Actually, I think these are Nurse Sharks and they are supposedly harmless. On my 40th birthday I went on a snorkeling trip off northern Belize. Absolutely stunning reefs with lots of different fish and rays. But at the end of one day on the reef, as it was getting cloudy and the water dark, nurse sharks started to circle around in the distance. We were told they were harmless but I scooted back up on the boat lickety-split.