Trivia: A video game from Sega in 1972 was called Killer Shark, considered the first video game in the horror genre. The object of the game was to shoot at a giant shark with harpoons. This game can be seen in the seaside arcade in the movie.
Trivia: Often touted as the first "summer movie;" what this refers to is the now-typical marketing practice of doing most of the advertising before the film's release to build up hype ahead of a big nationwide opening weekend (at the time most movies opened in a limited number of markets to build a reputation). By the 80s this model was followed by big tentpole "summer blockbusters," but nowadays is the norm for the majority of studio releases year-round.
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