Bay of Sighs (The Guardians Trilogy Book 2) By Nora Roberts

A dark and powerful goddess hunts for three jewels: fallen stars that will give her endless power. To save the world, six friends have joined forces to stop her. Now, on the beautiful island of Capri, their battle continues... Mermaid Annika is proud to have been chosen for such an important quest. But now that her identity has been revealed, her time is running out. She knows that soon she must return to her people. But she also knows that she is in love with Sawyer King ...