Any Duke in a Storm by Amalie Howard (Daring Dukes #4)

Historical romance takes to the high seas. Famed spy Lady Lisbeth Medford is on a ship bound for the West Indies, but the only thing more dangerous than her mission is the elusive Duke she's trying to capture. Lady Lisbeth Medford, Countess of Waterstone and famed international spy, is caught between the devil and the deep blue sea. She's determined to infiltrate a notorious smuggling ring in the West Indies while on a covert mission as a ship's captain. But even when her identity is compromised and she's forced to flee, the men chasing her are still hot on her heels. The trouble in front of her, however, might be even worse. Raphael Saint, the Duc de Viel, is her ship's new aggravating and dangerously charming sailing master, who might very well be part of the smuggling ring Lisbeth must bring to justice. But when a new deadly threat on the high seas looms, the only way out of danger might be to face it… together.