Quote from Jellomob
About the Water dimension, we discussed a bit about that a few pages back and decided on an idea I came up with based on the underwater caves that have large circular entrances. They are found throughout a certain part of Mexico I bekive.
Basically, the entire water dimension is covered in a thin layer of crust, the crust is covered in dense jungle and is littered with these huge holes that serve as entrances to the vast ocean that lies underneath the thin crust. The places where there are breaches in the crust receive light from the surface of course, but everywhere else is mostly dark. However, bioluminescent jellyfish are in vast in number providing plenty of light.
Also @hollowknight: I think I understand your thought process, but when you think about it, tropical oceans harbor much more life than cold ones. Plus the entire idea was inspired by caves in a mexican jungle, so I think it fits better.
As a jellyfish I assume you'll be right at home there.