First let's look at a shark. Can you find his nose? Yep, it's that incredibly narrow point in front that drops precipitously into a slimy, downward sloping ramp right into his 5,000,000-toothed mouth.
Take into account the fact that you're bobbing around in the water like a buoy full of meat, facing off against a lightning-quick predator with several million years of practice in eating things that punch it in the nose, and you'll see that chances are your fist will just deflect down into that aforementioned gaping hole of teeth like Boba Fett into a Sarlacc Pit. Now if your plan to defeat the shark is too feed yourself to him until he grows tired of the taste of you, you're off to a great start.
But experts say that even if you hit the shark-nose punching lottery, you won't have scared him off, just dazed him, giving you a minute or so tops. Now, if this particular shark is suffering from ADD, then great: You're in the clear now. He's off to chase a shiny thing or update his twitter feed 82 times an hour. But if not, all you've done is managed to piss the shark off and give him a minute to ponder how revenge is a dish best served in a blood-filled bag resembling you.