Aquaman is, for all intents and purposes, a marine mammal. And, with the exception of a healthy mane in later incarnations, he is effectively hairless. As a human, we would expect his internal body temperature to hover around 99°F, or about 37°C. Even at its warmest points, the surface temperature of the ocean around the equator is only about 80°F/27°C. At the poles ocean temperature can actually drop a few degrees below freezing. In the deep sea, ambient temperature levels out around 2 – 4°C. The ocean is cold, and water is a much better thermal conductor than air. Warm blooded species have evolved many different systems to manage these gradients, including countercurrent heat exchangers, insulating fur, and heavy layers of blubber.
That's one thing, but there are plenty of other reasons ocean living isn't all that pleasant. Link -via Not Exactly Rocket Science