Here is a concept drawing for an abyssal fish lady, they inhabit deep waters and dark caves. They do have bioluminescence in their bodies to illuminate their paths and to attract prey.
I still haven't decided if i should add the ability to swim in ROAG, there are many ideas i have involving water levels, but introducing swimming now would feel weird considering how much water where you can't swim there is in the game as of now.
I might add it to the successor of ROAG. You'll be able to swim in Crania Souls eventually, of course swimming in the underworld is considerably more dangerous.