There was a little rash of these "nature survival horror" films to come out in the early 2000s. This one and Frozen are my two favorites. But you could argue this one started the trend. A couple on vacation in Australia are taken out deep sea diving and, due to a Home Alone style counting accident, they are abandoned by the tour boat. Miles from land, the two find themselves in shark infested waters trying to survive. As a ticking clock, we get scenes of the tour owner finally realizing he is short two people. Will he get back out in time to save them? Even if he does get there in time, will both be alive?
These kinds of movies are intensely frightening to me. When you are a tourist somewhere, you assume that the people taking you into dangerous places are professionals who would never let harm befall you. However, these professionals are humans capable of mistakes as well and one bad day could leave you and your loved ones vulnerable. The heart of this movie is the couple, who were already having marital issues before they got stranded. It doesn't take too long for panic and anxiety to settle into the blame game and then you introduce some hungry sharks and you have a recipe for horror.