So I’m running a couple of 5th Edition D&D campaigns right now, one in my apartment, one over Roll20.net. (I highly recommend Roll20 to anyone who want to play D&D online, by the way.) This is the world I created for the online campaign, Itasoa. It is, to say the least, very nautical-themed. I originally dreamed it up for other purposes, but converted it to a D&D campaign setting, which necessitated converting the cosmology into a more D&D compatible universe (hence the references to the Feywild, the Shadowfell, etc.), and replacing some of the non-human sentient species I had with D&D species.

Itasoa is an ancient rogue world, spinning alone and adrift in the depths of space. Light and heat come from hundreds of tiny suns closely orbiting the world, and night is almost unknown, except rarely at the poles and in the depths the sea, and occasionally during powerful storms. The surface is almost entirely submerged by a single globe-spanning ocean. The majority of habitable surface on the planet is on the backs of the Godshells, enormous crustaceans miles across that never submerge, with island ecosystems forming on their back as they stride slowly through the oceans.
Geography:
Itasoa is almost entirely covered in an enormous ocean. There are only two continents- one a little ways down from the equator, and a larger one in the northern polar regions. The larger continent, Hurdun, in the north is ice and snow bound, and about the size of Great Britain. The smaller desert continent, Trine, about half the size of Hurdun, is only lightly populated, thanks to the strange, unearthly ruins filling the hot, arid interior, and the mysterious rocklike beings roaming them. There are also a moderate number of volcanic islands scattered throughout the ocean, though they’re created via the lifecycle of the Godshells, rather than tectonic activity, which is relatively sparse and slow on Itasoa. Water-breather civilization is frequently built up the side of the islands, with air-breathers atop it.
There are also massive kelp mats stretching for miles upon miles- these are largely inhabited by amphibious races, and the occasional barbarian tribe. They are prone to breaking up in godwaves and mightier storms.
Some sections of the ocean floor are riddled with mighty, immense rifts filled with horrifying, incomprehensible beasts.
The higher seafloor between rifts is still immensely deep and pitch dark, and is home to monsters, civilizations of insane, blind, voracious creatures, and the remnants of uncounted ships and rotting Godshell corpses.
There are a few Sargassos in Itasoa- current-less areas with little wind, and infrequent visits from Godshells. Getting stranded in one is not recommended.

Megafauna:

• –The Godshells: The Godshells have an extremely strange lifecycle, one upon which the entire ecosystem of Itasoa is dependent. The massive Godshells bury their eggs deep beneath the seafloor, away from any potential threats. The eggs incubate for centuries, until they launch themselves from beneath the seafloor, out of the oceans, and into orbit around Itasoa. This frequently also results in the birth of volcanic islands. They then burst into flame for several more centuries, before extinguishing and crashing back down into the ocean and hatching into a new Godshell, producing massive godwaves, like towering ripple rings a hundred miles across. While they are in orbit, most maintain an equatorial orbit, or a staggered orbit taking them from one tropic to another. Relatively few maintain a polar orbit, leaving the poles quite cold, with dim lighting, though actual night is still almost nonexistent.
  Godshells stride about the oceans on numerous enormous limbs- the frontmost of which also serve as digging limbs. They only support some of their weight with their legs- the majority is supported by their massive internal float bladders, keeping their backs afloat in all weather. They do avoid continental shelves and deep sea rifts, however. They feed on algae, plankton, and trace minerals. Almost all subspecies have natural harbors formed by indents in their shells, most commonly on the sides or back of the shell. They move relatively slowly, rarely clocking in at above a mile per hour, choosing their paths carefully to avoid deep-sea rifts, continents, erupting volcanoes, and depending on the subspecies, the territory of others of their kind. Godshells live for uncountable millenia, but do eventually die and sink to the bottom, though the dwellers will generally get plenty of notice, as foul miasmas boil up around it, and the shell shakes and loses stability, and the plants atop it begin to die.
  Most of Itasoa’s population lives on the Godshells- air-breathers on their backs, water-breathers on the parts of their shells that reside underwater. The area in between, where the sun, storm, and moon tides move the water about, is usually uninhabited, except by daring children and trash collectors during the appropriate tides.
  Godshells are the source of all magic- arcane magic is drawn from the sun-eggs, while divine magic is drawn from the adults. Though many worship the Godshells, it is unknown whether they are intelligent or not- many of their worshippers even believe them beasts, albeit divine ones.
  Godshells are ecosystems unto themselves. Their bodily waste is the center of entire ecosystems- they are followed by absolutely uncountable swarms of fish. Coral reefs frequently grow upon the submerged sections of their shell. Soil gradually builds up on their rough shell, creating entire land bound ecosystems as plants and trees root to the shells. Biomes ranging from jungles to grasslands exist upon their backs, though deserts are fairly rare. Plants and animals frequently transfer between neighboring Godshells, whether by swimming, flying, or cross-pollination and seeding.

• Subspecies:
  • Highstrine- The tallest of the Godshells, its nearly sheer sides stretch many hundreds of feet in the air. The only easy way up the top lies up the long, shallow slope running up the back of the Highstrines. The flora and fauna atop Highstrines tend to be relatively sparse, with the soil on the top occasionally as thin as a couple of feet over the shell. The section stretching above the water averages around seven miles long and five across. About a hundred feet under the waves, the sheer sides branch out into half mile shelves where aquatic races dwell, along with along the long slope in the back as it stretches underwater.
  • Vudren- A relatively small subspecies, averaging only about three miles wide and three miles long above the water, they’re notable for the natural valleys formed by the upward curving horn-like structures running front to back along their shells- protected from storms, the cities inside are some of the safest, most peaceful places to live, at least from natural threats. Rather solitary.
  • Chuldusintrus- Mid-sized subspecies, notable for their terraced shells. They average five to six miles in each direction. Travel in archipelagos of four to seven, though one will rarely split off to travel to the colder regions near the poles for a short time while the others wait for it in warmer water.
  • Mellitis- Four to six miles across above the water, and two to three miles long, they stretch for another eight outwards at a shallow slope underneath the water, making them by far the most favored of the Godshells for water-breathers. Their backs also reach the lowest heights of any godshell. They travel in large archipelagos of thirty or so, with members frequently joining and splitting off. The water in the midst of the archipelago tends to be fairly calm, thanks to the sheltering effect of so many Godshells.
  • Sabreck- Sabreck shells actually jut above the waterline in two noncontiguous islands, each about two miles across, with about that much distance between them. Their shell drops precipitously around the outside, forcing water-breathers to dwell between the islands. Highly territorial. Though they ignore most other subspecies, Sabrecks will not enter each others’ territory except for mating. Inexplicably, they will occasionally flee from Vudren.
  • Xiatan- By far the largest of the Godshells, and one of the rarest. Their shells grow up to forty miles across, far beyond the size of any others. The underwater portions stretch outwards another five miles in a moderate slope. The enormous, shallow-sloped spines on their back come to resemble mountain ranges as soil fills in around them. They do not go out of their way to avoid or approach each other outside of mating season, and other, smaller Godshells frequently follow behind them.
  • Sturnshells- The smallest Godshells, they only grow up to a mile in diameter, with half mile sections underwater. They’re frequently mistaken for juveniles of other species.
  • Durindas- Their shells stretch for up to fifteen miles miles front to back, and twelve miles side to side. They move somewhat faster than other species, occasionally being clocked at two miles per hour. They will willingly approach closer to rifts than any other species. Their underwater shell. extends for nearly five miles. Durindas are less heavily populated than would be expected by their size- the close approaches to the rifts are extremely dangerous for both water- and air-breathers. Water-dwellers are frequently attacked and eaten by the dwellers, and the periodic entrances into deeper waters mean that permanent settlements cannot be built on the lower air slopes. Nevertheless, the desperate frequently flock to Durindas- whether to find someplace they cannot be found, or to risk their lives and ships for wealth, hunting rift dwellers.
• Physalids- Rare, immense flying jellyfish-like beasts. Their gently sloped inflated bodies grow up to a half-mile in width, and the tentacles grow up to ten miles long. They denude the seas below them of life as they float along in the wind. Mighty spellcasters have been known to build habitations, towers, and even academies atop the shells. During storms, they attract lightning bolt after lightning bolt to their tentacles, which they spread out in an immense, almost globular pattern to keep aloft.
• Devilmaws- The only living predators of the Godshells. Any smaller subspecies or juvenile that drifts too close to an undersea rift is at risk of being pulled wholly under and torn to shreds. Only glimpses of the Devilmaws have ever been seen from the submerged portions of the mightier godshells, most frequently Durindas. They are immensely long and segmented, and move at astonishing speeds. The appearances of their heads is completely unknown, except perhaps by the insane deep dwellers.

Civilization:

• Most Godshells bear city states upon their back, though some of the larger may bear multiple cities- whether united or independent varies. Technology lies around a Renaissance equivalent- guns exist, but are one shot at best, and wildly inaccurate. Cannon are considerably more common, being far more reliable. City states range from democracies to monarchies to theocracies to anarchic trade cities, to everything in between.
• Floating wooden cities do exist, but are relatively rare.
• Races- Most races are divided evenly amongst the Godshells. There are very few city-states dominated by a single race, though it’s certainly not unknown. Many races play similar roles to the ones they normally play, though adapted to Itasoa.
  • Dwarves reside primarily upon volcanic islands and the two continents, though there are certainly many to be found in the Godshell city states. The dwarves are notable for traveling in stone ships entirely made of pumice- though slow and wallowing, they are immensely sturdy and powerful.
  • Gnomish-style trade fleets have become disproportionately wealthy as of late, thanks to their early and enthusiastic adoption of cannons. (Gnomish-style fleets refer to their organizational structure- they are owned and crewed by all sorts, though gnomes do own and operate the majority of these fleets.)
  • Elves only live around five centuries. The different types of elves mingle fairly freely. The closest thing to an Elven homeland is the Forest of Iav. Renowned as some of the best sailors, many elves spend as much of their lives at sea as possible.
  • Humans are everywhere, albeit in smaller populations than usual- rather than making up the majority of the global population, as in most settings, they’re merely another average sized population group.
  • Halflings are everywhere, and have one of the largest populations of any race on Itasoa. It’s considered bad luck to sail without at least a halfling cook. (It’s also bad luck to sail without a bevy of ship’s cats, a pebble imbedded in the stern rail, and a number of other things.)
  • Orcs are vicious, swarming sea raiders. Though frequently called barbarians, their ships are well maintained, fast, and sturdy, and they’ve begun manufacturing their own cannons. Half-orcs are viewed with as much distrust as in any other civilization.
  • Tieflings are received much more warmly on Itasoa than on other worlds. Their tails serve them well in the riggings of ships, and whatever pact gave them their form happened uncounted millenia in the past, before the Outer Planes, both Infernal and Celestial, withdrew from Itasoan affairs. Their population is much larger on Itasoa than other worlds, though they still number less than some of the other races.
  • Half-Elves are very common. No one looks at them twice.
  • Dragonborn are the rarest of the races, residing almost solely in their ice cities in Hurdun, though more have begun traveling the seas in recent years.

Notable Locations:

• Xiasadrik– The single largest Godshell on Itasoa, it stretches over one hundred miles in diameter, reaching miles above the sea. Though it was formerly inhabited by a number of squabbling city states, in recent years it has been consolidated by Atreus Silvertongue, a young and bloodthirsty elven demagogue turned tyrant.
• The Everstorm: An unending, slowly drifting and raging hurricane. A few sailors who have survived it claim to have seen a single, mighty Godshell of unknown species in the eye of the storm.
• The Boil- A region of ocean where three godeggs fell into the ocean in incredible proximity within the last couple centuries. The water for miles around boils from the enormous volcano building upwards underneath- already a massive, barren spire has worked its way above the surface.
• Necropoli- For some unknown reason, the shells of dead Godshells occasionally float back up from the ocean floor, whether for hours, days, or sometimes weeks. They are almost always covered and filled with disturbing, twisted architecture, built by the mad, blind sea-dwellers. Great rewards await those who brave them, but at immense risk.
• The Maw of Itasoa- an immense whirlpool that forms at the same spot at irregular intervals, lasts for a ten of sleep cycles, and then vanishes with an immense geyser of water. Big enough to devour and destroy any Godshell that approaches.
• The Forest of Iav- A forest of immense trees that climb from the continental shelf of Trine, stretching hundreds of feet above the water. Their trunks and branches have many strange, porous tunnels that fill and empty with the tides, and many of them have cities built atop them. There is a large population of manatees there, since the merfolk that live amongst the roots forbid their hunting in the region. There is a disproportionately large population of elves in the branch cities.
• The Rocks of Celemdun- Assuming a vessel can traverse these jagged rock spires jutting from the ocean, filled with treacherous currents and terrifying inhabitants, they come to the Pool of Celemdun- an immense, ancient Kraken. It is inscrutable, capricious, and possessed of a malevolent sense of humor and an unmatched talent for divination and prophecy. Ships that make it all the way to Celemdun may be rewarded with the answers to their questions, so long as Celemdun remains amused by the questions, and isn’t hungry.
• The Great Arch of Hurdun- A five mile long stone arch that stretches from the slightly warmer southern Hurdun coast into the sea. Surrounded by comparably sized, immense parallel stone ridges, many of which are well on their way to eroding into arches. There is a small city near the base that thrives off of the frequent visitors.
• The Sunken Moon– An immense underwater stone sphere, stretching nearly up to the surface. It is riddled with a labyrinth of caves, and it is said that a great treasure waits any who can make it all the way to the center.
• The Giarandiva Current- A mighty, powerful sea current capable of transporting ships at immense speeds. It is by no means safe, however- many poorly built ships have foundered in it, and it is patrolled by pirates, jealous trade fleets, and hungry sea serpents.
• The Whale Runs- Huge patches of sea filled with hundreds, or even thousands of pods of whales and dolphins. A dangerous place for ships, though only via mishap- the whales are generally unaggressive, except towards whalers, sea monsters, and sharks, who are occasionally foolish, hungry, or greedy enough to approach. Since Itasoan whales can grow much, much larger than our whales, the encroachers don’t generally have very long to regret their mistake.
• That Vile Island- A large floating island jutting out of the ocean, apparently entirely fashioned of seaweed, kelp, etc. It is, however, filled with noxious oozes and disturbing aberrations, on top of being entirely poisonous itself. Some sailors claim there is more than one, drifting on sea currents.
• Haumea- A relatively large Mellitis Godshell, those who live on it are filled with a sense of peace and well-being. Violence is rare amongst inhabitants, and fish and seaweed nearby grow in unprecedented amounts.
• The Mad Sturnshell- This normally small, timid species of Godshell has produced a bizarre, aggressive beast. It travels far swifter than other Godshells, frequently topping five miles an hour in rapid sprints when it spots other Godshells. It will physically ram itself against Godshells many times its size, and occasionally tries to submerge itself or lift itself above the water with very limited success, though it does at least avoid rifts and other serious dangers to its life. Other Godshells, even Xiatans, will flee it when they can, usually when it has exhausted itself. It does not attack juveniles. Needless to say, it has no known inhabitants, above or below the waterline. It has been known to sink ships, though almost certainly not on purpose- it’s doubtful it even notices. The Madshell has been acting like this for a bit over a century.
• The Temple of Haeran the Mad- Haeran was the king of a mighty city-state centuries ago. One day, a visitor gave him a strange gift- an Orrery depicting Itasoa without the godeggs orbiting it, and orbiting around a single sun with other worlds. Haeran became obsessed with it, eventually falling into madness. He led his people off their Godshell, and onto a nearby island, where they constructed a massive temple. On the day the last carving was completed, everyone but Haeran vanished entirely. Centuries later, he is still said to wait on his throne, though most visitors never return.

Cosmology:
There are no gods as most worlds would recognize them on Itasoa- though the planes can be accessed from it, albeit with slightly more difficulty, gods, archfiends, and archfey alike avoid Itasoa like the plague, though they will not say why. At every spot where access to Itasoa from the planes is easy, the planes have massive, floating stone menhirs- whether it is the Feywild, one of the Elemental Planes, Tartarus, or any other plane reachable via planar travel. All of the menhirs are covered in runes in an unknown language, and all are identical in structure.
Though most inhabitants are likely to participate in Godshell worship, there are secretive Rift cults, who worship the fell beings of the seafloor and the hideous, gargantuan inhabitants of the Rifts, such as the Devilmaws, and even deeper dwellers. Their secretive lore also mentions something referred to only as “The Deepest”.
The souls of Itasoan dead are not taken to the Outer Planes- the menhirs do seem to prevent that. The goodhearted are taken to reside amongst the burning sun-eggs, in a strange demi-plane of lush fields, gentle hills, and babbling streams. Those who have lived evil lives, however, have their spirits dragged down and chained to the seafloor for eternity, which likely helps contribute to the madness of the deep civilizations. The truly evil, those who have done horrific and fell deeds in life, however, are dragged all the way into the Rifts. It is unknown what happens to them there.