A fish to the head....
Imagine this guy (the fish, not one of the guys holding it) surprising you from out of the water next to you and before you can make out what the fuck it is, it slams into the side of your head, potentially knocking you unconscious or throwing you from the boat. But, as said by one of the quotes in the article, "it's not an attack so much as a collision".
http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2007/06/17/wfish117.xml