During one scene in JAWS 2 the teens' fear and cries of shark were real, as they were being circled by a 15-foot shark. The production crew at a distance thought they were acting and just gave them a thumbs up.

Marc Gilpin (Sean) on JAWS 2

From the interview sessions of The Shark is Still Working documentary available on the JAWS Blu-ray. Featuring JAWS 2 actor Marc Gilpin.

Source: https://www.youtube.com/watch?v=J9oMZAFSlxo