Open Water (2004)

An American couple on an island holiday, board a local dive boat for an underwater tour of the reef. Due to innocent miscommunication and a distracted crew, the couple is, after only 40 minutes or so underwater, accidentally left behind. What follows is the story of their ordeal: cold, alone and miles from land, the couple is adrift in shark-infested waters.

Official website: http://www.openwatermovie.com