During a late-night beach party on Amity Island, a young woman goes swimming in the ocean. While treading water, she is violently pulled under. The next morning, her partial remains are found on shore. The medical examiner ruling the death a shark attack leads Police Chief Martin Brody to close the beaches. Mayor Larry Vaughn overrules him, fearing it will ruin the town's summer economy. The coroner now concurs with the mayor's theory that the girl was killed in a boating accident. Brody reluctantly accepts their conclusion until another fatal shark attack occurs shortly after. Amid an amateur shark-hunting frenzy, local professional shark hunter Quint offers his services for $10,000. Meanwhile, consulting oceanographer Matt Hooper examines the first victim's remains and concludes the death was from a shark attack.
When local fishermen catch a large tiger shark, the mayor proclaims the beaches safe. Hooper disputes it being the same predator, confirming this after no human remains are found inside it. Hooper and Brody find a half-sunken boat while searching the night waters in Hooper's boat. Hooper examines it underwater and retrieves a sizable great white shark's tooth embedded in the hull. He drops it after finding a partial corpse. Vaughn discounts Brody and Hooper's claims that a huge great white shark is responsible and refuses to close the beaches, allowing only added safety precautions. On the Fourth of July weekend, tourists pack the beaches. Following a juvenile prank, the real shark enters a nearby estuary, killing a boater and causing Brody's oldest son Michael to go into temporary shock. Brody finally convinces a devastated Vaughn to hire Quint.
Quint, Brody, and Hooper set out on Quint's boat, the Orca, to hunt the shark. While Brody lays down a chum line, Quint waits for an opportunity to hook the shark. Without warning, it appears behind the boat. Quint, estimating its length at 25 feet (7.6 m), harpoons it with a line attached to a flotation barrel, but the shark pulls the barrel underwater and disappears.
At nightfall, the three retire to the boat's cabin and swap stories. The great white returns unexpectedly, ramming the boat's hull and killing the power. The men work through the night repairing the engine. In the morning, Brody attempts to call the Coast Guard, but Quint smashes the radio, enraging Brody. After a long chase, Quint harpoons another barrel into the shark. The line is tied to the stern, but the shark drags the boat backwards, swamping the deck and flooding the engine compartment. Quint severs the line to prevent the transom from being pulled out. He heads toward shore to draw the shark into shallower waters, but the overtaxed engine quits, immobilizing the boat.
With the Orca slowly sinking, the trio attempt a riskier approach: Hooper dons scuba gear and enters the water in a shark-proof cage, intending to lethally inject the shark with strychnine using a hypodermic spear. The shark demolishes the cage before Hooper can inject it, but he manages to escape to the seabed. The shark then attacks the boat directly, crushing the transom and devouring Quint. Trapped on the sinking vessel, Brody stuffs a pressurized scuba tank into the shark's mouth, and, climbing the mast, shoots the tank with a rifle. The resulting explosion obliterates the shark. Hooper resurfaces, and he and Brody paddle to Amity Island clinging to boat wreckage.
Output: What does Hooper find while searching the boat sunken underwater?