Protip! Don't Kiss A Shark On The Mouth

Diving guide Dave Marcel had been taking tourists out on Elbow Reef off Key Largo, Florida for years - allowing them to interact with the reef's wildlife including the 300-pound nurse shark.

These sharks are not usually aggressive, and love being petted.

Marcel would prove their harmlessness by kissing the sharks, and then allowing the tourists to do the same. 

Until the day one of the sharks bit back.