From the description:
"Elizabeth Valchar-pretty, popular, and perfect-wakes up the morning after her eighteenth birthday party on her family's yacht, where she'd been celebrating with her six closest friends. A persistent thumping noise has roused her. When she goes to investigate, what she finds will change everything she thought she knew about her life, her friends, and everything in between. As Liz begins to unravel the circumstances surrounding her birthday night, she will find that no one around her, least of all Liz herself, was perfect-or innocent. Critically acclaimed author Jessica Warman brings readers along on a roller-coaster ride of a mystery, one that is also a heartbreaking character study, a touching romance, and ultimately a hopeful tale of redemption, love, and letting go."
Readers like this one, too: 4.4 stars from 48 readers, only one negative review. A good sized young adult novel (474 pages).