Brody, Hooper, and Quint set out on Quint’s boat, the Orca to hunt the shark. During their pursuit, the trio form an unlikely bond of friendship and trust. After several unsuccessful close encounters with the shark, they decide to use Hooper's anti-shark cage. Hooper tries luring the shark close enough pump lethal drugs into it with a hypodermic spear, but the shark destroys the cage. Hooper escapes unharmed and dives to the bottom for safety. The shark then attacks the boat, damaging it so bad it begins to sinks. As seawater rushes into the stern, the Orca tilts upwards. Quint loses his grip, sliding into the beast's mouth and is killed. Knee-deep in water and sinking fast, Brody makes a final stand. He tosses one of Hooper's scuba tank into the shark’s mouth and climbs onto the ship's mast. He fires his rifle directly at the tank, blowing the shark to smithereens. Hooper surfaces, and he and Brody swim safely back to shore.
Demonhunter
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