Cursed in Love by Bethany Shaw

Guardian Ethney Wallace's job is to eradicate the lycanthrope threat before they can feast on unsuspecting humans in Savannah. Gifted with the ability to turn into a mermaid, she fights alongside the other eleven guardians. It's not an easy job, but it's one she enjoys, at least until innocent children get hurt. On a solo mission gone wrong, Noah, a stunning stranger who has been infected by the lycanthrope, rescues her. He and his young niece, Claire, need Ethney's help, but she's been down this road before and it has never ended well.
Noah Hawthorne and Claire survived a brutal lycanthrope attack, but they didn't come away completely unscathed. Both were bitten and now risk becoming the very monsters that murdered their family. With the full moon two weeks away they are in a race against time to stop the mutation from spreading within them.
Together, Noah and Ethney search for a cure, which brings them closer. Resisting the budding attraction between them is arduous; a romance is forbidden, as Ethney is honor bound to destroy all lycanthrope. If she and Noah fail in finding a cure, she will have to destroy him.