The Overworld is one ocean that goes down to y -704, and your base is a boat you build block by block

I got tired of ocean content being a biome you cross on the way to somewhere else, so I spent a few months making the ocean the whole game.

There is no other world type. Every new world is open sea with small tropical islands in it, and you spawn standing on a houseboat with no land in sight. The world floor sits at y -704 instead of vanilla's -64, so there is 640 blocks of extra water under you, and the sea floor has its own terrain down there: trenches, ridges, seamounts, flat abyssal plain, kelp forests, flooded blue holes. Water color is computed from the real depth of the column you are looking through, so it goes turquoise, blue, navy as the ground falls away, and the underwater fog closes in until the sky is gone.

The houseboat is the part I spent the longest on. You craft it from five logs around any boat, which gives you a helm on a 3×3 deck, and from there you build on it with whatever you want up to 32 by 32 by 16. Furnaces keep smelting while you sail. A bed on board is your respawn point wherever the boat ends up. You can walk around on it while it moves, place and break blocks underway, and several people can ride at once with a helm each. Mine the helm and the whole thing comes back up as one item.

The large jungle islands have hut villages on them, built out of 22 hut designs I made by hand, each with a dock and a boat shed. The nine villager professions are renamed and retextured around island life, and since jungle is the only tree that grows, every other kind of sapling is something you have to trade for. A Wayfinder sells charts to wrecks and ruins.

For things that want to eat you: four species of shark that circle and wind up before they charge, a giant squid down in the trenches, blue whales in pods that surface to blow and sing, and jellyfish and crabs that are harmless. Shipwrecks, ocean ruins, monuments, strongholds, ancient cities and trial chambers all still generate, the caves under the sea bed are sealed and dry so you can actually mine, and I pulled out ruined portals and buried treasure because neither made sense out here.

It is Fabric only, 26.3, and it needs a new world since it replaces the world type entirely. Screenshots are with Packtrix and AstraLex, but the water color, the depth fog and the bioluminescence at night are the mod's and show up without shaders.

I'm happy to answer anything about how the terrain or the boat works.