It absolutely was a Garand. Lockbar rear sights and everything. Someone even said that was Roy Sheider's personal firearm but I don't know if that is actually true.
https://www.milsurps.com/images/impo...sGarand5-1.jpg
They also used it in the scene when the Coast Guard guys pointed guns at the kids wearing fake shark fins.
https://www.milsurps.com/images/impo...sGarand1-1.jpg
Here is more of the firearms in Jaws.
http://www.imfdb.org/index.php/Jaws