47 Meters Down

Short version:
Lisa (Mandy Moore) finds Kate (Claire Holt), who has been badly injured and is nearly out of oxygen, and manages to drag her to the surface where, despite several shark bites, they are pulled into the boat. It is then revealed that Lisa was hallucinating and is still trapped in the shark cage 47 meters down. Kate is presumed dead. The Coast Guard finds Lisa, frees her, and guides her up to the surface as the credits roll.

Long version:
Lisa (Mandy Moore) and Kate (Claire Holt) are sisters vacationing in Mexico. They meet two men who invite them out to see great white sharks from a cage. Lisa doesn't want to do it (and does not have SCUBA certification), but as a skilled SCUBA diver, Kate promises to take care of Lisa. 

They meet the guys on the boat of the captain, Taylor (Matthew Modine), and go out on the water. The guys take a turn without a problem, but when the girls get in the rope breaks and they plummet 47 meters down to the bottom of the ocean. 

As they are out of communication range, Kate has to swim a couple of meters above the cage in order to talk to Taylor through her suit's comm unit. Taylor promises to send his employee, Javier, down with a spare cable to pull the cage back up, but when Kate and Lisa finally see his flashlight it does not move in their direction. They assume he can't find them. 

Since Kate is very low on air, Lisa swims out to find Javier. She locates the flashlight but Javier is nowhere in sight. Lisa loses her way and begins to panic when Javier yells for her to hide--just in time for a shark to kill him. Lisa manages to hide until all seems calm. She eventually crawls out, locates the cable to pull them up and a spear gun, and Kate guides her back to the cage by making noise when Lisa shines the flashlight in her direction. 

Kate's air is now perilously low and Lisa is getting down there as well. Lisa swims up until she is in range of Taylor and informs him that Javier is dead but they have the cable. Taylor pulls the cage back up but as they are getting close to the surface the cable breaks and they plummet back to the bottom.

Lisa's leg is now trapped under the cage. Kate swims up to tell Taylor they are still alive and he promises to drop down extra air tanks while they wait for the Mexican Coast Guard, which should arrive in about an hour. Significantly, Taylor warns Kate that switching to a second tank increases the risk of nitrogen narcosis, which causes hallucinations. 

When the tanks are dropped, Kate swims out to retrieve them and finds Taylor has sent flares as well. Kate switches her tank and narrowly makes it before running out of oxygen. Just as she arrives back at the cage, a shark swoops in, grabs her, and they both disappear. 

Lisa's replacement tank has fallen just out of reach. She cuts her hand on the spear gun trying to use it to pull the tank toward her on her first attempt. She keeps trying and manages to pull the tank to her just as she runs out of air. With no oxygen, she struggles to get it attached and finally does, breathing in relief. 

Shortly after, Kate begins talking to her through the comm system and tells Lisa that she is alive but badly injured and her blood is attracting more sharks. Lisa realizes she can use her BCD to lift the cage and free her leg. She finds Kate and begins to help her swim up toward the surface. Kate reminds Lisa that they cannot swim quickly or they will get the bends and die, and Kate helps Lisa pace them as they swim up--using the flares to scare off sharks as they approach. They get into range of Taylor and he reminds them to stop at the twenty-meter mark so they can adjust to the pressure to avoid the bends. While they wait, Kate accidentally drops a flare and when she manages to ignite another the sharks have surrounded them, so they swim up to the surface immediately and make a break for the boat. 

They catch a life preserver Taylor throws out, but a shark grabs Lisa and drags her under. She fights it off by gouging its eye with her fingers, and the sisters are hauled into the boat. As Taylor and the guys administer CPR, Lisa looks at the wound on her hand from the spear gun. It is still bleeding, but the blood is rising out of the wound as if she were underwater. It is then revealed that Lisa is still trapped in the cage, having hallucinated the whole escape. She is still hallucinating when the Coast Guard team finds the cage and pulls her toward the surface. As they approach the boat, Lisa seems to realize Kate is not with them and begins to cry.

Thanks Greg B!