Tidal

The third book in the addictive Watersong series, Tidal is a dramatic adventure by Amanda Hocking, author of the bestselling Trylle novels. Gemma is facing the fight of her life. Cursed by beautiful but deadly sirens, her extraordinary powers have a terrifyingly dark side, and becoming human once more is proving her greatest challenge yet. As she struggles to break the curse, the sirens Penn, Lexi and Thea are determined to kill her before she can set herself free.
Gemma's only allies are her sister, Harper, and Harper's boyfriend, Daniel. Together they must delve into their enemies' mythical pasts - to discover their darkest secrets. But Penn has also set her sights on Daniel. Soon, the sirens threaten everything Gemma holds dear: her family, her friends, her life, and her relationship with Alex - the only guy she's ever loved. Can she save herself and those she cares about before it is too late?