Where richard Dreyfuss is dropping bait over the side of the boat and the shark comes over the side, his face was a picture!!!!! , that is tense,!!!!!!! Though maybe that is shock for the first time viewer not tension, when the clever reverese camera shot pulls brady's face towards us
For me, I think it was in the very beginning when you first see the swimmer (Chrissy) getting pulled down. Because it's all dark and you don't know when the shark is going to attack.....
Some of the bits where the shark submerged and they didn't know when it was going to reappear
The guy trying to swim back to the wrecked jetty
Some of the early beach scenes when you know there's going to be an attack
Paul Shone's book Blockbuster contains a graph of audience reactions of Jaws - the biggest being the severed head, a scene filmed hurriedly long after the main shooting had ended.
(Pedantic point - it's Roy Scheider who's "chumming" when the shark rears out of the water and who then makes the "bigger boat" comment.)