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.
October 24, 2024
Make sure you also change the biomes!
If you don't change this, the islands will become ocean biome. This range should be similar to the one in the previous image.