It's just a hoax. There are two kids with a cardboard fin.
6 seconds sound clip from the Jaws (1975) movie soundboard.
You can hear this line at 01:01:20 in the Blu-ray version of the movie.
Quote context
[...]
- I want them out of there, guys. Give an answer, please. What's going on out there?
- He made me do it. He talked me into it.
- It's just a hoax. There are two kids with a cardboard fin.
- Is everyone there okay? Did everyone get out of the water all right?
- Folks, we had some practical joker down there with a fake fin...
- Shark! A shark! It's going into the pond! Shark in the estuary!
[...]