The Heiress at Sea by Christi Caldwell

A seafaring journey is the second chance for a lady and the last chance for a marquess in a thrilling novel about rebellious love, secrets, and danger by bestselling author Christi Caldwell.

Perpetually unattached Lady Cassia McQuoid has accepted her fate as a spinster. But how can she be expected to lead a life confined to drawing rooms and royal balls when there’s a vast, exciting world to explore? Cassia can find but one solution: she masquerades as a thrill-seeking lad and stows away on a ship bound for open waters. Adventure awaits. So does notorious privateer Nathaniel Ellsby.
Nathaniel doesn’t want to be the Duke of Roxburghe. Instead, the second-born son relishes his freedom to do as he pleases: pursuing enemy ships in service of the war effort. But when his older brother dies, Nathaniel embarks on one last mission before he fulfills his ducal duties with an arranged marriage. That’s the intention, at least—until he discovers that his woefully green deckhand is a fetching but vulnerable woman in disguise.
Nathaniel’s new mission is to protect Cassia from the perils of the sea, both above and below the water, by sharing the close confines of the captain’s quarters. As roiling emotions turn to love, Nathaniel and Cassia realize the greatest risks—to the Crown, to their futures, and to the heart—still lie ahead.