Dark Water by J.A. Armitage

You thought you knew the story of The Little Mermaid… You were wrong.
A tragic accident on the eve of her eighteenth birthday leads Princess Erica to a chance encounter with a mysterious man on the beach. When he turns up at her birthday party and whisks her off her feet, she knows she has to find out everything about him, but why won’t he speak and where does he keep disappearing to?
Ari knows he’s crossed a line by mingling with the land humans, but how can he resist the beautiful princess whose life he saved? He must decide whether to stay on land and become like her, or continue to live without her in the depths of the ocean.
Trapped in a centuries long battle between his kind and hers, his decision will rock both their kingdoms.
With a sea witch that needs repaying and the small matter of Erica’s engagement to someone else, Ari knows the odds are stacked against him.