So I was just watching The Green Hornet  and I got to the scene where Seth Rogen and Jay Chou fight.
Seems like the inability to swim is an easy weakness to give to otherwise super bad-ass characters. They can do almost anything and everything requiring athletic ability with ease except swim. And that's when the "main" character can jump in and save them, showing that they really can do something this other person can't. Otherwise, you'd think to yourself, how come this one isn't the main character? I'd rather watch him in his day-to-day life, not this wimpy hack. Or, in this case, I still think that. I wanna see The Kato Show, not The Green Hornet. *tsk tsk*
Exhibit A: The Green Hornet 
Kato is a genius with martial arts, mechanics and latte art but never in all his years did he learn how to swim on a planet that's 71% covered by water? Nawww....
Exhibit B: Pendragon Series by D.J. Machale
Two separate bad-ass characters, Loor and Kasha, both prove physically superior to the main protagonist, Bobby, in every respect except swimming. Bobby saved them each at least once with his special ability.
Exhibit C: Deltora Series by Emily Rodda
Jasmine, again a female who is much more physically capable than the male lead, needs saving whenever the adventurous gang falls into some body of water.
That last one was from a children's book series you might not have encountered. Nevertheless, this trope exists and I find it annoying to say the least. But then maybe that's just me. I say make 'em some kind of ~phobic if you must. At least that's more believable, but no less cheap, than simply never learning to swim. Unless that character came from a desert or something. Then it's believable.
P.S. The title of this post is a lie. I'm an awesome swimmer. (And most humble, too) (O_O)-b