We Were Liars by E. Lockhart

Cadence and her family have spent every summer together on their family's private island. They are wealthy, beautiful, and seem to have everything. Cadence, Mirren, Johnny, and Gat, the three cousins and a family friend are inseparable. Everything appears to be perfect until the incident. Cadence can't remember what happened and no one will tell her how she she ended up down by the water alone with no memory. When Cadence returns to the island after missing a summer, she tries to uncover what happened.

I can't comment too much on this one because I do not want to ruin anything. It is definitely at the top of my list of books to recommend. The best I can do is tell you to go read it and avoid any reviews or summaries.