1. bears are pretty good in land and water (esp. polar bears), so they could technically go to land to "recharge their batteries" and then come back totally ready to go in water. sharks are limited.
2. bears have more sharpness than just teeth, i.e. more weaponry (claws)

...the only thing that throws me off is that sharks are really good at smelling blood, so once the shark drew blood from the brear, more sharks would follow and the bear would be at a disadvantage.

this would definitely be an interesting fight...and i'm still skeptical about the "arena" such a brawl would/could be held in. hmmm....