Between by Jessica Warman

Between is the amazingly gripping story that proves nobody is as innocent as they seem, and some secrets will haunt you until your very life ends.
On the eve of her 18th birthday, the young and beautiful Elizabeth Valchar is celebrating on her family's boat with her 5 closest friends - Richie, Josie, Caroline, Mera and Topher.
After a gathering of booze and weed, everyone falls asleep. Everyone except for Elizabeth, who is kept awake by the constant sound of something banging against the yacht. When she can't stand it anymore, she picks her drunk self up and heads out onto the deck of the boat.
At first, it seemed like nothing, but further investigation reveals Elizabeth's worst fear imaginable - death.
Before Elizabeth can decide anything, a certain, but familiar, stranger appears before her, holding the key to every secret that she has and to every memory that needs to unlock.
Jessica Warman has built this book to every suspense imaginable, giving readers the absolute feel that they are there, helping Elizabeth on her journey to discovering the true about almost everything in her life.