Fish (noun): the term occasionally used on a channel crossing or a marathon swim by an experienced support crew for the type of fish with a cartilaginous skeleton and a highly streamlined body with a tough usually dull gray skin, commonly known as a shark. Sharks are typically active predators that fascinating and frightening, found in all seas, ranging in size from 17 cm to 12 meters, with a few species sometimes dangerous to humans.
The support crew saw the fish near the swimmer, but did not point to it in order to avoid scaring the swimmer. Synonyms: Shark, The Man in the Gray Suit, Mack, Old Toothy, Garbage Can of the Sea. [Some scholars believe shark is derived from the German word schurke, meaning villain.]
Excerpt from the Open Water Swimming Dictionary (2009 American English version).