A group of friends wakeboarding are attacked and devoured by a two-headed shark. Meanwhile, a Semester at Sea ship, called the Sea King, led by Professor Franklin Babish and his wife Anne hit a large shark, that becomes lodged in the boats propeller, which damages the ship's hull, causing the boat to take on water. Soon after, the two-headed shark attacks the boat and breaks the radio antenna, preventing ship co-captain Laura from summoning help. The group notice a deserted atoll near-by, so Laura drives the Sea King closer, before Professor Babish takes the students, consisting of Kate, Paul, Laura, Liza, Cole, Lyndsey, Ethan, Jamie, Ryan, Kirsten, Alex, Kristen, Kirk, Haley, Jeff, Alison, and Dana, over to the atoll on a dinghy.Anne remains on the Sea King, along with Laura and the ship's crew, Han and Dikilla. Laura enters the water to repair the ship's hull, but is quickly ripped apart by the two-headed shark. Meanwhile, the group searches the atoll for scrap metal to help repair the boat, where Kate tells Kirsten she is afraid of the water. Kirk, Haley and Alison decide to go skinny dipping, only to be attacked and eaten by the two-headed shark. The rest of the group meet up and find two small speedboats before an apparent earthquake hits, causing Professor Babish to fall and badly cut his leg. Jeff and Mike take Professor Babish back to the Sea King on the dinghy and Anne tends to her husband while Jeff and Mike head back to the atoll, and discover Laura's severed hand in the water before being attacked by the two-headed shark. They attempt to swim back to the Sea King, but both are ultimately killed.Kate and Paul manage to fix the two boats they found, while Cole finds a gasoline tank to fuel them. Cole then jumps in one of the boats with Ryan, Jamie and Alex and drives away, prompting Kate, Paul, and Dana to follow in the other. The two boats race each other, unaware of Professor Babish's and Anne's warnings of the two-headed shark. The two-headed shark attacks Cole's boat, causing Ryan to fall into the water, where he is devoured, notifying the other students of the shark. Paul realizes the shark will continue to chase Cole's boat as it has a bigger engine. Cole also realizes this and jumps off the boat, leaving Jamie and Alex to be eaten. Kate, Paul and Dana save Cole and arrive back at the atoll, followed by Professor Babish, Anne, Han, and Dikilla on the dinghy.The group creates a plan. They hook up a generator to metal poles and place them in the water to distract the shark while Kate and Cole travel to the Sea King and repair the hull. The plan works until the two-headed shark attacks the poles, knocking Han and Dikilla into the water where they are eaten, before the two-headed shark swims to the Sea King. Kate fixes the Sea King, only for Cole to drive away without her, forcing her to swim back to the atoll. The two-headed shark attacks the Sea King, causing it to sink, resulting in Cole being devoured and the boat sending out a distress signal. As the group panic, anothe earthquake hits and the atoll begins to rapidly sink, causing the group to realize that the supposed earthquakes were actually the atoll collapsing in on itself from the underground. Kristen and Dana become separated from the group and are devoured, as are Professor Babish and Anne. A small tsunami hits and the atoll sinks, leaving the survivors with little land. They take shelter in the still standing chapel, but the shark breaks in and devours Lyndsey, Michelle, Liza, and Ethan.Kate, Paul and Kirsten discover the gasoline tank and lure the shark towards it. Kate attempts to stab the two-headed shark, but Kirsten attracts its attention and attempts to blow it up with the tank with her lighter, but she is eaten. Kate and Paul find one of the boats, and turn it on, before taking shelter on part of the remaining atoll. The shark attacks the boat, which explodes, killing the shark. A helicopter then comes to save Kate and Paul, the only survivors of the entire group of 23 people.
Output: Where did the survivors take shelter?