This post contains lots of spoilers!!
Shy is from a poor neighborhood in Southern California who gets a job on-board a luxury cruise ship. His grandmother just died from a mysterious new disease. He's made new friends on the cruise ship, including Carmen, who he's secretly in love with even though she's engaged to a guy back home.
On his first cruise, a guy commits suicide right in front of him after talking about what an awful person he is. On his second cruise, there's a creepy guy in a suit that is watching him and eventually confronts him, wanting to know what suicide guy told him before he died.
A huge storm hits the ship. During the storm they get word that California has been hit by a massive earthquake. Really massive - off the Richter scale. And that means tsunamis. The ship is hit by several tsunamis. Shy is separated from his friends but survives, even though many people die. He eventually makes it to a raft with a few other crew. Their raft is hit by another tsunami and Shy abandons the raft. He happens upon a lifeboat that has been badly damaged. Everyone inside is dead. He's able to patch the hole in the bottom but it won't start. He comes upon a sinking raft with a few people left. And there are sharks circling. Several don't make it, but two are able to make it to the lifeboat. However, one of them is bitten and eventually dies. Before dying, he slips Shy a huge diamond ring that he was planning on proposing to has girlfriend with. The other person that makes it to the lifeboat is a spoiled teenage girl named Addie. They fight at first but eventually learn more about each other and come to like each other (and Addie seems to kinda fall for Shy, but he's still pretty hung up on Carmen, hoping she's still alive).
They have one oar and use it to go in the direction of some islands that other passengers have talked about. Addie's dad works for some secretive medical company on the islands. They come across an abandoned motorboat with two dead scientists on-board and a duffel bag with unmarked medications. The controls are destroyed so they stay with the lifeboat, but Shy takes the duffel bag with them.
They are eventually rescued by another motorboat with Shoeshine, an older crew member from the cruise.