Terrors of the High Seas by Melissa Good

After the stress of a long Navy project and Kerry's father's death, Dar and Kerry decide to take their first long vacation together. A cruise in the eastern Caribbean seems just the nice, peaceful time they need to unwind and relax. It is not long, though, until they get involved in a family feud, an old murder, and come face to face with pirates as their vacation turns into a race to find the key to a decades old puzzle.