Roar of Neptune

Ayra has never been able to sit still, not when she hears the siren's call of the ocean. She's been drawn to it ever since she can remember - and before being adopted by her older mother, she doesn't remember much. Everything is a blur. During one volatile storm, Ayra gets swept up by vicious waves who pull her under. But instead of drowning her, they save her. They take her to a palace with no ruler except a tyrant who has turned the people into slaves.
But one thief recognizes Ayra. He claims she's the heir to the throne. The only hope for these underwater people. In order to save them, Ayra must unleash her power and be brave enough to claim what is hers. If she doesn't die trying first.