During the scene where the two boys fool everyone using a "cardboard fin," all of the swimmers rush for shore in a panic, thinking there is a shark in the water. If you look closely, you can see several people laughing, including one of the men waving everyone in.
actually when jaws was filmed they didnt pay the people in the water/ the life guards, so they werent actors, and they often only did one shot so they left it in, the people in the back didnt know it was a movie.