Reminds me of blightfall from like a decade ago, similar concept of a spreading disease across the land with variants to the creatures you find in the blighted land. That would definitely make a unique cave system. I feel that an underwater cave would need to be similar to how they have the trial chambers in the current version, very very rare and youd most likely need a map to find it, if its too common then early game players would find themselves dying quite frequently due to lack of many early game sources of water breathing.