Black Waters (Book 1 in the Songstress Trilogy)
Sixteen-year-old mermaid Abby Carson thought she had her secret under control. Who cares if after “the change” took place, she became so beautiful that all the girls at school hated her. And the boys, well, when they look at her, hate isn't exactly what's going through their minds. But when Abby witnesses a teenage girl's murder by a preternatural creature off the coast of Cape Cod, her carefully controlled life slips into chaos. Hiding in the water until the killer's gone is a must. Getting spotted by Brian Baker, a boy from school, isn't. Neither is falling in love with him. That definitely wasn't part of the plan. Combine all this with the mysterious mermaid box Abby receives as a postmortem birthday gift from her dad, and she's pulled even deeper into the world of shades and monsters. Once the full force of Abby's deadly siren powers are revealed, the only one she can hurt more than herself is the boy she loves.