Marlena Frank

A battered castaway, a mysterious girl, and a sea witch lurking in the shadows. Out on the ocean catching crabs with his father, seventeen-year-old Finn would rather spend his summers on a boat than head back to school in the fall. When a bad storm blows in, Finn's life changes forever. Barely escaping with his life, Finn washes up on an island with a lonely lighthouse, but he isn't alone. A teenage girl named Ash lives at the lighthouse.
She has always lived there, always been alone, but what happened to her? Who is she, and why does she hide at the top of the lighthouse each night? As the two teens fall in love, they reveal secrets, unravel mysteries, and unearth the lighthouse's frightful past. A monstrous sea witch stalks the shoreline. She waits just outside the door, lurks behind the windows, and waits for them to make a mistake.
Her shriek makes their blood run cold. Finn and Ash must escape the witch and the island, but they are running out of time. Finn is desperate. He will do whatever it takes to save the love of his life. Even if it means losing her forever.