Can anyone help with the name of this movie?

A few teens go off to an island (I think by boat) and one by one they are murdered. The only survivor is a girl who recounts the story (through her eyes) at a police station and says she can't remember bits and twists other stuff. It turns out at the end she is the murderer and was saying her name was 'blah blah' but actually turned to to be someone else. Any Ideas?