How to create a ocean/underwater biome

What continentalness value did you try?

-1.2 to -1.05 should generate your biome similarly to the mushroom fields

1. You need to understand how biome continentalness works and what biomes use what
   1. Ocean biomes use -0.455 to -0.19 and Deep Ocean biomes use -1.05 to -0.455, -1.2 to -1.05 is mushroom fields and will always generate as an island.
   2. You need to understand that your biome, if generated properly, will generate only as deep as the deep ocean biome as it is not possible without directly influencing minecrafts world generation and biome generation to cater to your needs which I don't recommend unless you have advanced skill sets in json, java, and you know what perlin noise is.
   3. If you are OKAY with this fact, then proceed.
2. Biomes fight over a spot in the overworld if they share similar value, the biome with the bigger range values will almost always overtake the biome with the lower range of values so in order to do an Ocean Biome that only generated in Deep Oceans, you'll have to accept the size of your biome may not be to your liking.
3. Set your Biome Temperature to generate the biome connected to your preferred deep ocean biome.
4. Set your Humidity to -1 to 1
5. Continentalness needs to be -1.05 to -0.6 to ensure it only generates within deep ocean biomes (this will make your biome smaller)
6. Erosion is not hugely important with ocean biomes, however, I always set my ocean biome erosion high. If you want to increase your odds of your biome spawning, increase the range between min and max values
7. Weirdness: set this to -1 to 1, it wont do anything aside from reduce the size or chance of your biome generating with lower ranges.
8. Test your biome.
9. Notes:
   1. Yes your biome will be smaller than the typical ocean biome, nothing you can do with your preferences
   2. There is still a chance it will attach to normal ocean biomes, however, it will never contact land