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.
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.