Introduction:

So after 10 years of playing me and my friend wanted to spice up things a bit and play in an ocean only world where the whole gameplay would be different since the different available resources. We didn't want to choose between all the available ocean biomes like frozen deep ocean or warm ocean since some of them are very different in terms of default blocks such as coral or ice.

Changes since 1.21.5 release:

• Reverted back to stock world generation: Added back top layer of ice in most snowy/frozen biomes to make it easier for the player to navigate
• Added Underwater Potion to most village chests that grants the player 30 minutes of water breathing upon drinking
• Corals, budding amethyst, ice variants, mushroom blocks can be mined with simple tools without silk touch enchantment
• Reverse crafting ice, corals, mushroom blocks via crafting table or stonecutter is now possible
• Safe world spawn is still risky. Since I'm still horrible at coding and couldn't come up with a fix platform generation during the first time the player spawns in the world, I'll give you pre-tested seeds that have safe world spawn:

• Spawn on ice: 906182347964544722
  

• Spawn on abandoned ship: 6696165159819216322
  

• Spawn on a floating nether portal: 2589681248334255595
  

Changes since 1.21 release:

• Set water limit to 208 as well as generated terrain.
• Most structures will generate in the world just fine, including villages, outposts, mansions, swamp huts, igloos, strongholds etc. but for some reason jungle and desert pyramids won't?
• Dimensions were left untouched, only noise settings and the biomes were modified.
• Removed ice layer on top from all biomes to remove way too much free, flat surface from a water themed survival world to avoid too easy gameplay. I'll add different editions for enhanced gameplay, such as modified villages, mansions and outposts to fit a water themed world.

Changes since version 1.18 release:

Mojang/Microsoft drastically changed world generation and while there are more options for fine-tuning the vanilla gameplay they removed/altered several stuff so making custom world generators is still an unknown field. Also they've removed custom structure and structure start support so you cannot just tell the game to generate 10 biomes but put all sorts of structures such as villages, strongholds, mansions etc. on top of it. Meaning, structures are now hard-coded and will only generate in their respective biome. Dark oak forest->Mansion. Villages cannot generate in ocean biomes. I had to solve this issue and there weren't many options for me. I increased sea level from 63 to 284. Telling the world to generate ocean biomes only was no longer an option for you'd be missing almost all and every structure including villages, outposts, swamp huts, buried treasures, mansions etc. Imagine it this way: The world is normal, every biome is there it's just that they're submerged like in the movie called Water World. According to my knowledge only desert and jungle pyramids are unable to generate this way and I'm unable to solve that. There are a few issues with this sudden increase of sea level and submerging biomes though. For some reason the player won't care anymore if it's a safe place to spawn or not. You can get lucky and the world will put your default spawn at an iceberg on top of the ocean or mansion, village, outposts etc. but you can also get buried in a cave suffocating or at the bottom of the sea 200 meters below... This needs fixing but at the current world generator settings I cannot do anything to fix this... So there are a few workarounds for this and I'll talk about them in the features section.

As of 1.18:

• Every biome generates now. They're just submerged. 
• Sea level increased from 63 to 284.
• You can play with sea level by modifying /data/minecraft/worldgen/noise_settings/overworld.json file in the data pack before generating the world and editing "sea_level": 284" to your desired value.
• Safe spawn in no longer an option. You can spawn on the bottom of the ocean or in a stone wall or a cave or if you're lucky on the top of an iceberg, village, mansion, outpost etc. Thank Mojang for this.
• Most structures still spawn with the exception of desert and jungle pyramid and thankfully they're still above the sea so you can find them relatively easily.

Workarounds of getting default player spawn to a safe place:

• Enter the world in creative and find a safe place then use /setworldspawn command to place your default spawn to a safe location.
• Keep generating new worlds with the data pack until you get a safe spawn location. If you're patient this could be best for you if you really don't want to use creative mode.

Up to 1.17.1:

• The world only generates ocean biomes such as: Ocean, Deep Ocean, Frozen Ocean, Deep Frozen Ocean, Cold Ocean, Deep Cold Ocean, Lukewarm Ocean, Deep Lukewarm Ocean, Warm Ocean, Deep Warm Ocean
• Villages as of 1.0.1 plus edition generate in their respective ocean biomes. Plains village in ocean, savannna in lukewarm, desert in warm, taiga in frozen ocean.
• The world still generates most structures (if allowed to) such as: ocean monuments, desert pyramids, villages, pillager outposts, ruined nether portals, shipwrecks, igloos
• Sadly strongholds and mansions don't generate and I'm not good enough to make them do so
• Now that I rebuilt the data pack from scratch, not based on other sources strongholds, mansions, jungle temples, buried treasures and all sorts of villages can generate too!
• The player can set the wanted ocean depth before starting a new game by modifying sea_level line in overworld.json in the extracted zip files \data\minecraft\worldgen\noise_settings location (default is 67 blocks. 222 is a very deep sea...

Different versions:

Based the work on slicedlime Mojang developer's vanilla worldgen and nate101thebattler's Water World Datapack. (the old one, which you can silll download from the files. Beware, that version doesn't generate strongholds so the end is unavailable)

Remade from scratch with the help of misode's data pack editor.

Older versions:

Here on Modrinth you can download the basic edition which only generates oceans with strongholds, buried treasures and ruined portals. More challenging and recommended gameplay. There is the plus edition which generates all sorts of structures such as various villages, depending on the ocean type, mansions, jungle temples, pillager outposts, desert pyramids. I've also made a resource pack that replaces the standard Minecraft overworld as well as the main menu music with more sailor like. You can download it in the files too.