Daughter of the Deep by Lina C. Amarego

A wedding to end a war. It’s been four years since the blood feud between the Branwens and Mathonwys started. Four years of sunken ships and loved ones lost, four years of wounds both seen and unseen. Keira Branwen is no stranger to duty. As the first mate of a legendary pirate ship, blessed by the sea-god Lyr, and heir apparent to her clan, she would sail to the Otherworld and back to keep her family safe. But when the Council threatens banishment for her people, Keira is faced with a choice; she must either marry her father’s alleged killer, the sea-snake Ronan Mathowny, or curse her family to the deep.

But in the Seven Isles of the Deyrnas, secrets sleep just beneath the surface of the water. Her father’s death is still shrouded in mystery, and Ronan—her childhood friend, the man she once loved — is hell-bent on proving his innocence. As enemies become friends and friends become suspects, Keira must wade through the waters of doubt to expose the truth, save her family, and restore her father’s legacy. But Keira soon learns that there are ties thicker than blood, and there is more than one way to sacrifice a life.