Continuity mistake: Throughout the movie we see a number of underwater scenes with the camera shot looking up under the swimmers where the water is very clear and blue given how many swimmers we can see from a distance. The only problem is that the water everyone is swimming in is greenish and cloudy as can be seen with the camera shots that are half in and out of the water right along side of the swimmers.
Continuity mistake: When Quint shouts for Hooper to tie up the first barrel, Hooper's grip on the electronic tracking device differs from one shot to the next. (01:24:15)
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