I love Jaws - it's one of my all time favorites, but this bit has always bugged me.
It's the scene near the end of the movie. The Orca and her crew are being chased by the shark. But it's still about 50 metres away, and doesn't appear to be gaining. Yet for some inexplicable reason, Quint turns the throttle up. Also you can see from previous shots that land isn't that far away, and it seems like if they just keep a narrow distance they should make it back to land before the shark can smash the boat.
In the original script the Orca's engine blows out because the shark is thrashing around on it, which makes more sense.
The Orca shudders from side to side. From Hooper's point of view we can the shark gripping the transom in his jaws, shaking the boat as he saws his massive head from side to side, trying to tear a chunk out of the very hull. Quint has reloaded and is firing into the fish. Brody has a wicked pointed gaff, and is swinging wildly at the snout, gashing and gouging it, trying for the eyes. The killing lust is on all three men.
QUINT Throttling back!
The boat surges, the shark gives a final unbalances wrench, and disengages. The dorsal fin circles off, beginning a wide loop around the boat.
The engine quivers and dies, the boat without power, rolling half awash, a wounded victim.
The fin dips, the barrels follow, the shark disappears beneath the waves. There is complete silence.