Revealing mistake: Just as Quint finishes scraping his nails on the chalkboard, the camera begins to move in closer when he takes the first bite of the cracker. On the right side of the screen under Harry's chin, Roy Scheider is ducking down while staring at the camera, as it pans toward Robert Shaw, and Roy then lifts his left hand up, cueing Robert's line. (00:20:55)
Continuity mistake: When Quint is up in the crow's nest, Brody tells him "Let Hooper take a turn" (regarding chumming the water). When Quint, while looking down at Brody, says "Hooper drives the boat, Chief", his right arm is across his body and looks to be resting on the safety railing of the crow's nest. When Brody looks up at Quint, his right arm is away from his body and is just holding onto the crow's nest railing with his hand.
Chosen answer: As mentioned in the movie, the posion was in the needle and the shark's hide was too tough for the needle to penetrate. Hooper had to go in the water so that he could get the needle into the shark's mouth, where the flesh was less tough.
Kevin Howard