Sinister Seas by Kira Nyte

Aria has shunned the human world since her love for a human prince brought fear and danger to her people. She believes she is safe under the sea—until a witch imprisons her family, steals her tail and sends her onto land on wobbling legs in search of the Forgotten Prince.
It’s been ten years since Caspian left the sea to make his way in the human world. Better that than lose his soul a little more each day to the princess who considers him nothing more than a feckless friend. But when she comes to him for help, he finds he is still helplessly under her spell. This time, he vows, he’ll weave a spell of his own.