The opah would like to differ on being cold.
This deep water fish has now been discovered to be an endotherm - just like us. It's not as much of an endotherm, but it does manage to regulate its body temperature to about 5C warmer than the surrounding water - even when at depth. Its a pretty big fish, too.
It does it by flapping its fins to generate and trap heat. And the advantage is, of course, that it can move faster than other fish in the deep water - all the better to eat them.
We do not know enough about the oceans. Let's do some more exploring.