I'm planning an encounter where my players will need to traverse a geyser-fed lake, where the water can range from uncomfortable to boiling, depending on the closeness to the nearest font. I'd like them to have to fight something that lives in these perilous waters (with the constant threat of falling into scalding water), but I am drawing a blank. Most of the monsters associated with heat are more acclimated to fire than boiling water or even highly humid environments.
Is there any published monster that would fit these criteria — aquatic, but able to resist high heat?
I'd prefer something from 5e, but previous editions are acceptable; I can adapt them. Also, I could easily just take any aquatic monster and give it Damage Immunity to fire, or any monster with Damage Immunity to fire and give aquatic adaptation. I'm looking for something that already thematically fits. Further, although extraplanar monsters are OK, I would prefer a "natural" or Prime Material Plane threat.
I know I could just give immunity to any existing 5e monster. That's simple to do, but I'd rather mechanically convert a thematically correct existing monster from a different edition than just make a boil-resistant hill giant.
My players are 5 characters of 7-8 level, but again, I can adjust.