There was this story around Mick Fanning this year. He was attacked by a shark in a surfing tournament.
In 2015, Fanning survived an encounter with what is suspected to be a great white shark during the J-Bay Open finals in Jeffreys Bay, South Africa.
It's unlikely to happen but theoretical you may find yourself in such a situation not only while surfing. This made me wonder how one should react to such?
Mick reacted like this (according to the Wikipedia article):
Fanning punched the shark and tried to wedge his board between the shark and his body. The shark eventually bit off Fanning's leash and Fanning started his attempt to flee back to shore.