I have had a hell of a time trying to modify the drowned biome at all. I played with its settings to try and get it to spawn ores, because that would have been an interesting use for it (making it a bit of a deep dark biome), and I got it to spawn grottos and dens but only too low for players to actually interact with them.

Since it's resisted all other efforts to give it a purpose, but I'd still like to use it, I think I'll edit in a few unique Pokemon to each Biome.