That's a porpoise
In 1965 I was watching the water show at the end of the Steel Pier at Atlantic City. The "Diving Clowns" were performing when someone in the audience spotted a fin. Soon the whole crowd was shouting "Shark!" I noticed that the tail fin was horizontal and not vertical so I started to chant "Flipper". The divers saw the fin and saluted the "Flipper" chant and continued the show.