Apparently, you're sposed to punch them on the nose becuase it disorients them.
I don't if this is true though.
(Applebite Uh! A cow!, Wed 18 Aug 2010, 12:01, 4 replies, latest was 16 years ago)

Not punch them, push them away by the nose,
they tend to bite things to see what they are, if they then bleed they'll eat you, so just push them away and they'll get bored.
(PsychoChomp, Wed 18 Aug 2010, 12:07, Reply)

apparently shouting at them works
although this is difficult underwater
( Vipros. clever got me this far, then tricky got me in, Wed 18 Aug 2010, 12:15, Reply)

"RAARRGLUG-GLUG-GLUGgaaaaaaaa"
*chokes and drowns*
(Applebite Uh! A cow!, Wed 18 Aug 2010, 12:17, Reply)

better or worse than being eaten by a shark do you think?

( Vipros. clever got me this far, then tricky got me in, Wed 18 Aug 2010, 12:19, Reply)

Better.

(Applebite Uh! A cow!, Wed 18 Aug 2010, 12:55, Reply)

or go for the eyes

( Amberl was stripey and dominated Europe, Wed 18 Aug 2010, 12:08, Reply)

While this might work for a small reef shark, the kind that wouldn't attack you anyway
it wouldn't work on anything large since a) they are too quick and would bite you before you could hit them and b) you couldn't get enough power into an underwater punch to upset them even if you did manage to get a lucky blow in.
(Bazongaloid, Wed 18 Aug 2010, 12:09, Reply)

It's really hard to punch underwater though
unless you're Angelina Jolie in the second tomb raider film
( Kitty v1.0 desperately naive, Wed 18 Aug 2010, 12:11, Reply)