Aquaman can shoot an endless stream of water, sink to the bottom, and has strength (which can be used underwater, too bad there's only one time you'll need that). All in all, more useful than the real Aquaman.
Something to note, in the Ace Chemicals level there are two puzzles where Hazard Suit Robin is useful that Aquaman isn't. Robin has to collect colored chemicals in his sprayer and shoot them into vats. But by the time you get Aquaman you can just turn into Superman and fly past the whole thing. Kind of a moot point, now that I think about it.
I read a ton of comics and I think Aquaman is a joke.
Somebody who rivals J'onn as the most powerful Earth telepath is a joke? There's a reason why no one ever wants to deal with Aquaman and always hires Manta: because if you piss him off, he will stop at nothing to find and kill you. After all, he has no such thing as a "no kill" rule and will put a trident straight through you if you so much as LOOK at Atlantis funny. He ain't the King of Atlantis for nothing.