You never heard of the attack manatees in blue springs state park? Apparently they attack people swimming there. They crush you against the side of the run, or between two adults and then continue to gobble you up. There are even warning signs: Beware the attack Manatee.
Many swimmers disappeared in the 70's when this was not known, then one manatee dies and they did an autopsy. What was found was startling: car license plates, coconuts, garbage bags and a human skull. After study, they found out that manatees change behavior after 7 years and become more aggressive, with the big bulls being the most dangerous. There are even stories of manatees hunting down snorkelers in packs, but I think that this is too far fetched.