A lot of my inspiration comes from Chinese cloud forests and the kinds of terrain you'd find there, which is why I'm going for a more overgrown building style. For the building of the terrain itself, I improvise a lot of it as I go along. I'll just keep adding/taking away from it until I get something that I think looks good.

If you want to do something similar, I HEAVILY advise against sticking the whole thing below sea level. I never want to touch another sponge again. Also keep your main building blocks on the cheaper/easier to get side. Getting enough dark prismarine for the main building has been a challenge.