If you make your character Aquaman, it is a pain in the ass for the GM to put a puddle of water in every situation just to give you something to do. He can't set an adventure in the desert. Superman in the desert is still Superman. Batman in the desert is still Batman. Aquaman in the desert is a useless chump.
If the GM is setting his campaign underwater or on the coast, you can (and maybe should) make Aquaman.
If the GM is setting his campaign in the middle of the desert or in outer space, you should consider making something else.
On the flip side: GMs, it is your responsibility to tell the players that your campaign is going to feature little to no aquatic action (or aquatic action all the damn time) well in advance.