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The values in this image tells the game what "continents" values the islands should generate. -0.95 to -0.4 correspond to deep oceans. Higher = more inland. -1 is around where mushroom islands generate though, as they are furthest from mainland.
If you don't change this, the islands will become ocean biome. This range should be similar to the one in the previous image.

Datapack that creates worlds with floating islands over the oceans for the default world.