Good, but needs something...
Your art is very good. He's got the basic muscular structure, he's in proportion, the coloring is great, and the fur looks furry. It's actually the pose he's in that make's this illustration look a bit impossible.
Look up some pictures of people swimming and you'll see what i mean. Also, it's very uncomfortable (and a bit silly) to have your arm up, with your forearm pointing down in a 90 degree angle. One last thing, if you're swimming, usually you want your hands splayed out so that it's easier to move.
I really like this piece, and I'd actually like to see you re-do it with a better pose, and possibly some lighting effects. :]