Legend of Mermaid Cliff by Brandon Rohrbaugh

Sixteen-year-old Mark Parker is devastated when he learns that his high school crush, Veronica McBride, has been found dead at the bottom of a cliff in their small beach town of Rockport. As the last person seen with the ill-fated girl, the town becomes suspicious of Mark and he swears to find the truth—but in doing so, he uncovers unearthly and evil secrets his town has been harboring for a century.