Jared (Paul Walker) and Sam (Jessica Alba) are two lovers living a rustic life in a trailer, next to the beach in the Bahamas. Sam works as a guide in the local aquatic theme park, "Atlantis", and Jared has worked a number of odd jobs in his field of passion, diving. His real dream, though, is finding one of many merchant and pirate ships lost in the waters around the Bahamas, many of them with large amounts of gold on board.
Jared's friend Bryce (Scott Caan) and his girlfriend Amanda (Ashley Scott) come to visit, as Bryce, a lawyer in New York City, has acquired the use of a luxury vacation house from a client he defended. While snorkeling, Jared comes across artifacts that seem to stem from a ship wreck. The four of them investigate and find several other pieces that turn out to be the remains of a legendary French pirate ship, the Zephyr, lost as the pirate made his getaway.
This legendary shipwreck is rumored to contain millions of dollars in gold. The foursome, thinking their dreams of discovering buried treasure have finally come true, decide to investigate. While exploring it to try and stake their claim, they come across the remains of a crashed plane carrying a large quantity of cocaine. Bryce and Amanda are advocates of keeping the cocaine to sell it for profit, while Jared and Sam oppose it. To avoid attracting attention to the location of their treasure find, Bryce and Amanda agree not to report the crashed plane and its illegal cargo until they have secured their claim on the sunken ship.
Jared and Bryce go to a local boat supplier in order to make a deal for the equipment needed to retrieve the treasure, with Bryce funding the purchase. However, Jared soon finds out that Bryce, during a trip to Las Vegas about a month ago, borrowed money from a loan shark and is in heavy debt and therefore is in no position to pay for anything.
Without Jared's knowledge, Bryce and Amanda come up with a plan to retrieve some of the cocaine from the sunken plane and sell it to local night club owner Primo (Tyson Beckford). Unfortunately, Primo turns out to be an associate of the drug lord Reyes (James Frain) to whom the cocaine belonged in the first place. Reyes, wanting his cocaine back, threatens Jared, Amanda and Bryce at gunpoint to bring him his cocaine or die.
Faced with this choice, Jared agrees to retrieve the cocaine. However, he tells Reyes that he will need $30,000 worth of non-refundable diving equipment to do so. Instead, he uses the situation to get to the gear he needs to salvage the Zephyr.
Back on land, the trio breaks the bad news to Sam. Sam is heartbroken that Jared would violate his principles and work for drug lords. He tries to convince her that it's okay and that it's a one-time thing, but Sam leaves him, saying that 'they' are over.
Jared, Bryce, and Amanda head to sea. They start the night's work by uncovering the site of the Zephyr, finding the ship's bell with the name on it. For Jared, the victory feels hollow, though.
As they are moving the cocaine packs from the plane to their surface boat, Amanda is attacked by a large tiger shark. She is brought to the hospital where she dies due to a huge bite out of her left leg. Hearing of the tragedy, Sam comes to the hospital and she and Jared are reunited.
While driving back from the hospital, the threesome (Jared, Sam, and Bryce) are confronted by Primo at an intersection. A short, but intense car chase ends up with Jared leaving their truck and telling Bryce and Sam to meet him later. He surrenders to Primo and tries to get them to back off by offering them and their boss the loot on the Zephyr. Primo still takes him to Reyes' boat. The boat looks deserted, so Primo tells Jared to lie down on the floor of the state room while he investigates. After some time, when Primo doesn't return, Jared goes to explore. He finds Primo, Reyes and several others shot dead in one the bedrooms. Out from the toilet comes Bates (Josh Brolin), a local salvager and former employer of Jared's. He asks Jared where "his cocaine" is and Jared bolts. He escapes from the boat and heads for shore.
Meanwhile, Sam insists on going to the police, and goes to the private home of one of their friends, a local cop named Roy (Dwayne Adway). Bryce drops her off, but thinking they'll get busted, heads off to meet Jared. Roy comforts Sam and tells her he knows a guy who can help, but that it has to be kept outside of the official channels, as he's not sure who he can trust. Roy betrays Sam and takes her to Bates, who captures her.
Jared and Bryce meet up and Jared calls the captive Sam, who is handcuffed and gagged with duct tape. Bates answers her cell phone and tells Jared to disclose the location of the cocaine. Jared insists that they meet and Jared show him to the location. Bryce and Jared then make a plan to settle the situation once and for all.
Jared is taken aboard Bates' boat, the Sea Robin. Here he is shown Sam, who is in the engine room, handcuffed to the body of Roy and gagged with duct tape, who was shot by Bates. The Sea Robin heads for the location where Jared directs them, but as they approach the area, they see a white substance floating on the surface. Jared tells Bates that it's half of the cocaine, ruined, and that Jared will destroy the rest. He jumps into the water, and Bates and his men, after changing into Scuba gear, go after him. Below the surface, in the plane, the divers look for Jared. Bryce has switched places with one of the dead pilots, and they use this to mount a surprise attack. An underwater struggle follows, killing all of Bates' men and wounding Bryce.
Meanwhile, Sam finds a forgotten machete in the engine room and uses it to free herself from Roy's body by cutting off his arm. She then severs a pipe, which brings one of Bates' men down into the engine room, where she locks him in. They then goes up to the bridge to call for help. The man escapes from the engine room; a chase around the boat ends in a fight that Sam ultimately wins as they go into the water and the man, wounded by Sam with a boat hook, is attacked by another tiger shark.
Below, Jared and Bates are the only ones left. Jared uses an air tank as a missile by hammering off the valve. Bates escapes it, but as it hits the fuel tank at the back of the plane, it causes the entire plane to explode, killing Bates. Sam jumps into the water, fearing for Jared's life, but he survives.
Six weeks later. The trio is salvaging the Zephyr. After trying to bring an old cannon atop fails and the cannon sinks back down to the bottom, Sam and Jared call it a night. However, Bryce sees something in the water, uncovered as the cannon hit the wood of the wreck's deck. He jumps in and dives down. As he comes back up he announces that he found the gold of the Zephyr.