|The opah, a warm blooded fish|
The fish lives in the deep ocean and is as large a a car tire. It's special adaptations allow it to generate its own body heat in the deep ocean. Most deep-water predatory fish are slow and wait for prey to come to them, but by generating its own heat the opah can move faster and chase down prey.
You can see more on the opah here.
If a fish like this lives in the deep ocean, why can't "sea serpents" live there, too?