In May 1962, on the Italian ocean liner Antonia Graza, dozens of wealthy passengers are dancing to the song "Senza Fine" sung by Francesca, an Italian singer. A young girl, Katie Harwood, is sitting alone, until the ship's captain offers to dance with her. Elsewhere, a hand presses a lever that unravels a thin wire cord from a spool. The spool snaps and the wire slices across the dance floor, bisecting the dancers.
Forty years later, at a bar, a boat salvage crew: Captain Sean Murphy, Maureen Epps, Greer, Dodge, Munder and Santos celebrate their recent success. Jack Ferriman, a Canadian weather service pilot, approaches them and says he spotted a vessel running adrift in the Bering Sea. Because the ship is in international waters, it can be claimed by whoever brings it to port. The crew sets out on the Arctic Warrior, an ocean salvage tugboat. The ship is the Antonia Graza, which was believed to be lost at sea. When they prepare to tow it, they discover it contains a large quantity of gold. After a series of supernatural events, the group decide to leave with the gold, but an invisible force sabotages the Arctic Warrior. The tugboat explodes as the engine is started, killing Santos.
Left with no other option, the group begins repairing the Antonia Graza. Greer encounters Francesca, who seduces him and leads him off a precipice to his death. Captain Murphy, after entering the captain's cabin, finds the ghost of the ship's Captain. The ghost explains that they recovered the gold from a sinking cruiseship, Lorelei, along with a sole survivor. Murphy is shown a picture of the survivor, whom he recognizes. He rushes to tell the others, but begins hallucinating and sees everyone as the burned ghost of Santos. The others think Murphy has gone mad and lock him in the drained fish tank.
Epps meets Katie's ghost, who reveals what happened on the Graza. The sole survivor of the Lorelei convinced the Graza's crew to murder the passengers, including Katie, for the gold. After the passengers are dead, one of the ship's officers turned on the rest of the crew and killed them, then Francesca killed him. Another man, the mastermind behind the massacre, then killed Francesca. The man is revealed as Jack Ferriman, who is actually a demonic spirit.
Epps deduces that Ferriman lured the salvage team to the Graza to repair it, and decides to sink it to stop him. She sets explosives, but is confronted by Ferriman, who has killed the last of her crew. Ferriman describes himself as a salvager of souls, a job he earned by a lifetime of sin. He plans to use the Antonia Graza as a trap and keep collecting souls. Epps detonates the explosives and sinks the Graza, and Katie helps her escape the sinking ship. She is left in the debris as the souls trapped on the ship ascend to heaven.
Drifting on the open sea, Epps is found by a cruise ship and returned to land. As she is loaded into an ambulance, she sees the battered crates of gold being loaded onto the cruise ship by Ferriman. Ferriman glares at her and carries on; she screams as the ambulance doors close.

Question: Why does the crew lock Murphy in the drained fish tank?
Answer: They believe he's gone mad.
Alternative answer: they think he has gone made