It's probably just a school of mackerel or something, all clumped together.
5 seconds sound clip from the Jaws (1975) movie soundboard.
You can hear this line at 00:46:51 in the Blu-ray version of the movie.
Quote context
[...]
- They pay a guy like you to watch sharks?
- Well, it doesn't make sense for a guy who hates the water to live on an island either.
- It's only an island if you look at it from the water.
- It's probably just a school of mackerel or something, all clumped together.
- There's something else out there.
- About 100 yards, south-southwest.
[...]