Heartsongs — Karolyn Cairns

The Honorable Lord Thaddeus Whitmore, the Marquess of Heath, and the heir to a great dukedom, was ship bound for the Caribbean, to beg the hand in marriage a foreign princess he neither wanted, or ever set his eyes upon. His dreaded errand was interrupted by pillaging pirates, sparing him a loveless marriage, and giving him the means of one last act of defiance against his noble sire.
Princess Leira of Mer came upon the horrific sea battle, hearing the legendary heartsong the minute she set eyes upon the handsome land dweller. Mer Legend foretold the melody was heard only once in a mermaid's immortal lifetime. She could neither ignore it and turn away as the ship began to sink under the waves. Determined to save him, and see her destiny to it's end, Leira follows her heart blindly, refusing to believe Tad isn't her one true love.
Leira must make a choice when Tad falls prey to the island's many dangers, and goes against the laws of her kind to save him. Now their destinies are entwined, whether she follows him into his world, or he stays in hers. Loving him as she does, she makes a choice, to give up her immortal life and remain with him.
Separated by a cruel twist of fate, Leira is forced to return to her underwater kingdom and atone for her crimes against her kind, imprisoned until she agrees to forget her land dweller. But Leira can't forget Tad, or the love that burns eternally between them. She vows to find him, and give up all for the belief they are made to be together.