Sharks simply don't like the taste of Humans.  Our blood does not attract them. Too many metals in it. In a study done both to captive and wild sharks, the results may surprise you. Human blood released attracted no sharks and resulted in no change in behavior.  Beef blood resulted in a mild curiosity of one shark in both captivity and the wild. Fish blood, however, resulted in frenzy like behavior.  You can watch a similar test on the Mythbusters - Drop of Blood episode.

Humans are rarely, if ever, devoured by a shark. Sharks use their teeth to test and decide if something would be a food source. Most "attacks" are not truly an attack but in fact a test bite.  Usually, the reason a shark will bite is simply out of mistaken identity.

60% of the encounters with sharks are surfers, boarders, and fishermen. 35% are bathers or swimmers, 5% are Divers.  Of the approximately 100 encounters globally annually, 4.2 deaths occur. In approximately 800,000,000 beach visits annually in the USA, we average 1 death every 2 years.