Amidst a storm, a boat is being piloted across the South China Sea by John Finnegan (Treat Williams). With him are his crewmen Joey "Tooch" Pantucci (Kevin J. O'Connor) and Joey's girlfriend Leila (Una Damon). Finnegan has been hired to transport a group of mercenaries, led by a man named Hanover (Wes Studi), to an undisclosed location in the middle of the ocean. Meanwhile, the luxury cruise ship Argonautica, the brainchild of Simon Canton (Anthony Heald), is undertaking its maiden voyage. Canton and the ship's captain, Atherton (Derrick O'Connor), discover a woman named Trillian (Famke Janssen) attempting to steal from the ship's vault. They lock her in a storeroom as the brig is incomplete. Soon after, a saboteur disables the ship's navigation and communication systems. A large object is then detected on the sonar, rising from beneath and headed towards the ship. It collides with the vessel, bringing it to a violent stop and leaving it dead in the water, while the panicking passengers begin to be attacked by unseen creatures.
Soon afterwards, Finnegan's boat collides with a speedboat shaken loose from the Argonautica, causing severe damage. It is revealed the mercenaries intend to rob the ship's vault and its wealthy passengers, before sinking the vessel with their torpedoes. The majority of the group transfer over to the ship while Leila and mercenary Billy (Clint Curtis) remain on Finnegan's boat to conduct repairs. The others explore the ship, finding only blood and damaged superstructure. They also begin to hear screeches and ominous sounds on the ship. Back on Finnegan's boat, Leila is violently dragged overboard, while Billy also disappears. Finnegan and Joey go to the cruiseliner's engine room to scavenge parts to repair their vessel’s engines, under guard of T-Ray (Trevor Goddard) and Mamooli (Cliff Curtis). Though the creatures kill both mercenaries, Finnegan and Joey escape with the necessary parts. They run into Trillian, who has escaped from the storeroom, and are confronted by the remaining mercenaries, who still intend to break into the ship's vault before leaving. Vivo (Djimon Hounsou) opens the vault, but is axed by Canton, who was hiding inside with Captain Atherton, as well as three passengers who are then machine-gunned by Mulligan (Jason Flemyng).
Canton and Atherton explain that the ship was attacked by unknown creatures that killed everyone else on board; the group soon discovers human skeletons stripped clean of flesh in the bottom of the ship. After questioning, Canton is also found to be responsible for sabotage of the Argonautica; having realised he created an unprofitable vessel, he hired the mercenaries to sink the ship so that he could collect on the insurance; the mercenaries' payment was to be by way of their robbery, while Canton insists the passengers would've been safely evacuated in the lifeboats. The group is suddenly attacked by the creatures, revealed to be giant tentacles, one of which is shot open, revealing a half-digested Billy. Captain Atherton is also killed, and the remaining survivors reluctantly agree to work together to escape. Canton theorises that the tentacles are an extreme evolution of smaller deep-sea worms which strip their prey of body fluids and then eject the carcass.
The group is forced to swim through a flooded section of the ship to reach an exit; Mason (Clifton Powell) is the last in line and is attacked, and he kills himself with a grenade. Once through, Mulligan goes insane and attempts to stand alone against the creatures, giving the others a chance to escape before he too is killed. It soon becomes apparent that the tentacles are herding the survivors to the bow, where they find a "feeding room" full of the passengers’ and crew’s carcasses. The tentacles then break through the hull and flood the lower decks, separating the survivors. Hanover tries to sacrifice Joey to save himself, but Joey escapes and Hanover is himself taken. Canton realises the ship is sinking and plans to abandon the others to their fate.
Finnegan and Joey spot an island in the distance and make it back to Finnegan's boat, but they have lost their engine parts, rendering it useless as a means of escape. They instead set the boat's autopilot to crash into the Argonautica and detonate the mercenaries' torpedoes. Finnegan returns to the ship to find jet skis they can use to reach the island, but is forced instead to rescue Trillian from Canton, who intends to eliminate her as she is a witness to his scheme. Canton escapes moments before the tentacles smash through the deck from below, revealing them to be part of a single giant creature, which attacks Finnegan and Trillian. Barely escaping, they return to Finnegan's boat, where they discover Joey gone amidst evidence of another attack. Finnegan sets the boat's autopilot and leaves the doomed cruiseliner on a jet ski with Trillian. Desperate to escape, Canton jumps aboard Finnegan's boat, but cannot disable the autopilot and dies after its collision with the Argonautica, the explosion destroying the entire ship and killing the creature.
Finnegan and Trillian reach the nearby jungle-filled island where they are reunited with Joey, who survived the ordeal and paddled ashore using Finnegan's surfboard. As the three relax, a loud roar echoes from the forest and something huge crashes toward them through the trees. The camera pans out to reveal multiple active volcanoes within the core of the island, while an exasperated Finnegan mutters, "Now what?"

Question: Why do Leila and Billy remain on Finnegan's boat?
Answer: To make repairs
Alternative answer: To conduct repairs.