When the girl is being attacked, she seems to do an incredible job of acting in pain. The thing is, she is not acting. The scene was done with divers under the water pulling the actress with ropes. They pulled the girl so hard that she broke several ribs.
People don't usually go to the ocean in the winter, but in Jaws, you will notice that everyone is in their swimsuits and bikinis at the beach, in the water, yet there are no leaves on the trees. The only reason that would be is if it was winter time.
Richard Dreyfus is driving the boat then playing cards then driving again.
There is a scene where Chief Brody is standing on the boat almost ankle deep in water with a pump in his hand. He throws down the pump and is about to climb up the ladder when (we presume) the shark hits the boat making it list to the right. When they show this scene again, not only is there no water in the boat, the deck is dry.
Chief Brody is at the wheel of the boat and Hooper and Quint are trying to fix the engine. Brody can only turn the wheel (in his words) about 3 inches. Just then some of the yellow barrels surface and they stop what they're doing to start the boat to chase the shark. The boat steers fine.
When the shark is somehow attached to some rope, Brody & Hooper tie the rope to the stern cleats, causing the boat to be dragged backwards. Quint gets a machete to cut the rope to limit the boat's damage. At this point the sea is somewhat choppy. He throws the machete making it stick into the edge of the boat. The sea in the background is completely calm.
In one of Richard Dreyfus' first scenes, Brody asks him to help with the group of men crowding into a small boat. Hooper says that the officer wants him to tell them that they're overcrowding the boat. They show the men climbing in and there's a fat guy in a blue jacket climbing down the ladder into the boat. He waves his arm and says "what do you care?" They then show Hooper asking if they know a good restaurant or hotel on the island. When they show the men again, uttering the line "Yeah, walk straight ahead." the man in the blue jacket is on the deck again and is again descending down the ladder.
Another scene that is proof that the film was made in either early spring or late fall is the scene where the mayor is arguing with Hooper and Brody about the existence of a shark in front of the graffiti covered sign. It is supposed to right before July 4th, yet everyone is in coats and jackets.
Quint comments to Hooper about the tranquilizer dart: "you gonna get that little needle through his skin?" looking at the hypodermic needle that Hooper is filling. Then later, the actual shark dart has a point far thicker, looks like it would pierce the skin of a battleship!