A Princess of Wind and Wave by Melanie Cellier

Isla may be a mermaid princess, but she’s never felt at home in her undersea kingdom. Instead she dreams of the day her people can return to the surface—and the boy whose life she once saved. But while Isla foresees danger for her people beneath the ocean, she has no concept of the dark forces working to keep her people submerged.
When an impetuous vow sets her on the course of the truth, Isla discovers that shadowy foes beset the kingdoms of both land and sea. And when she finally finds herself onshore, Isla realizes the fight will require not only her sword but her mind and heart as well—at least if she ever hopes to overcome her enemies and win the love of the prince of her dreams.
In this reimagining of the classic fairy tale, The Little Mermaid, the princess will have to grow beyond her dreams if she wants to save two kingdoms and her own heart.