You know? That's a really good question! Now I'm really thinking about it. XD I guess it all depends on if Flash knows how to swim. He could be one of those fail characters that totally sucks at swimming, and thrashes in the water instead. X'D Actually picturing him having a conniption in the water is somewhat hilarious.
But that would kind of suck for Aquaman if Flash could win because then people really will wonder if Aquaman has a purpose. That poor guy.
I'm assuming the density of water would at least slow him down if he can. Honestly, I think Aquaman has him beat. Though I have to wonder, is he showing him the fish or is giving it to him? I sense you might be insinuating something. *pointed look* X'D