Opens with a voice recording, and then a home video of a woman on a beach talking. It appears that she is part of a group of friends. They are drinking, laughing on the beach and generally having a good time. They are at El Matador beach, and the man recording says that these people are his best friends and that they are going to love each other forever. The home footage fades away and a "Based on true events" text appears on the black screen.Then it cuts to a driving car. Inside the car is Amy, James, and their baby daughter. The baby starts to cry so her parents start singing a lullaby and they drive down the coast. A Couple in motorcycle pass by them waving, they wave back as they realize the couple on the motorcycle are their friends Zach and Laura. There is general happiness as the credits scroll on happy Mexican vacation montage with lots of palm trees and shots of beautiful water.It is revealed that this group is a group of old high school friends-and the people from the home video footage from the beginning of the film. They are gathered in Mexico to celebrate Zach's birthday. Their other friend, Dan has invited them to sail on his brand new yacht to celebrate Zach's birthday. Amy is slightly hesitant to go on the water because she is scared of the water but steps onto the dock anyway with her baby. Amy makes it on to the boat, and they go down to the cabin and walk in on the captain having sex with a blonde woman named Michelle.Cut to montage of boat sailing fun. Nothing can go wrong when you are having fun on a boat. They are very hot and very drunk. Captain Dan reveals that the boat is brand new and this is the first time he has taken it out. Amy is still wearing her life jacket but Dan tries to persuade her to take it off. Michelle asks Amy why she is wearing a life vest and Amy lies about being afraid of the water by making a joke. Dan makes a big toast and congratulates them all for being high school best friends.A few hours later the partying has died down a little bit and Amy and Michelle talk about the lifeboats while the men sunbathe. Laura states that she is hot and that she wants to go swimming. Dan hands out snorkel masks, knives and a blow-up dolphin named Dauphine. Amy hears the baby monitor and goes down to take care of the baby. The others jump into the water, leaving Amy and Dan alone on the boat together. Dan watched Amy feed the baby on the bed. He teases her about not having any fun. She keeps life vest on as they banter back and forth and then they head upstairs after the baby falls asleep.The others are swimming in the water and practice their open water diving skills. Amy stays in her life vest on the boat. She admits to Dan that she is afraid of the water. Dan decides to play a joke on her, and then picks her up and both of them jump into the water. Amy begins to panic and starts having flashbacks of her dad drowning when she was a little girl. They realize that nobody put down the ladder so they are trapped in the water next to the boat. They all try to get back on to the boat but they can't get back up onto the boat. Michelle starts to complain that she is tired, and cold and doesn't want to drown. They try to use the blow-up dolphin to get on boat but that attempt fails too when the air starts leaking. Michelle starts getting angry and they start blaming each other. They try to grab the American flag off the boat. Dan grabs the flag but it rips and he falls back onto the water.Hopelessness falls upon the group. Amy wearing the life jacket is the only one not tired because she doesn't have to tread water. All of a sudden Michelle starts screaming that she feels something brush against her leg. They look under with their goggles but don' see anything.They start blaming Dan for not having a ladder. Now they realize that the baby is still up there and she is sleeping now but won't be for long. They start getting tired and try hanging on to each other but it's getting harder for them to stay afloat. They start praying as they realize that they are going to die.Time passes and they start getting cold and thirsty. They swim around the boat trying to find a way and it starts to get windy. They hear a cell phone ringing and try to get it. A jacket has fallen over the edge a little bit and they jump to get the jacket with the phone but the phone gets emerged in water. They answer the phone but the friends on the other side can't hear them and then the phone gets water logged and stops working. In a furious rage, Dan throws the phone away.One of them gets an idea to make a rope out of their clothes so they can toss it up on the boat and have it hook on something. They get naked and make a rope. Another boat comes their way and they try to wave it down but it doesn't see them and it thinks they re a party boat so they don't stop. They start making the rope with their clothes by cutting the shorts with a knife that Dan has. James takes another dive down and looks at the bottom of the boat. The rest of the group tries to get their rope tied around the rail but it is too heavy. The rope hooks around the railing and Dan tries to climb up the railing but he falls. James swims underneath the boat and drops the knife, he tries to grab it but he panics and hits his head on the side of the boat. His ear is bleeding very badly and his blood drips into the water. Laura says he has a skull fracture and he needs a hospital very fast. Zach takes the knife and starts stabbing the boat but Dan gets angry that he is stabbing his pretty boat and stabs him in the ribs. He starts bleeding and drops the knife into the water.The baby wakes up and starts crying. Amy panics even more, it seems that they are all doomed. Michelle swims away in panic and they can't find her. She must have swam away and then drowned. Dan goes to find her and sees her naked sinking body. He tries to save her but she is gone. Zach goes into delirium from drinking too much sea water and starts talking nonsense. Then he dies. Dan admits that he stole the boat and that his high roller life is all a lie so he stole it to show off to them. He says he is a fraud and that he is nothing. This is bad because now the coast guard doesn't even know that they are out so no one is looking for them. Laura starts washing blood off the boat, she says that she is going to try to swim back to shore because she isn't going to die dreading water. She gives them all really big hugs and there is a big tearful goodbye then she starts swimming towards the shore. The baby starts crying again and Amy panics even more.Now it is nighttime and the baby is still crying, and they haven't made it back onto the boat yet. The night passes slowly for them as they sit in the water next to the boat. James then dies in Amy's arms. Captain Dan keeps looking for the knife in the rain he dives deeper. She starts swimming away from the boat to find Dan. The captain reveals his love for Amy. Dan then breaks his swim goggles in half and uses the glass from the goggles to make a foothold in the boat. Amy climbs up and she pushes herself on deck. When she is finally on the boat she pushes the button and brings down the ladder.She goes into the cabin and sees that her baby is still alive. Dan slips and falls back into the water. Amy dives back in after him to save his life.The next morning a fishing boat pulls aside the sailboat. He looks for survivors but sees none and then he hears a baby crying. Amy and Dan both died at sea, but her baby survived.