Showing Duke By Day, Rogue By Night by Katherine Bone

Constance Danbury is desperate to escape an arranged marriage to a man nearly twice her age. Her only hope is to board a merchantman bound for Spain to enlist her aunt's help. Her plans go awry when she’s captured by pirates. Even more alarming, her traitorous body longs for the man who’s returning her to England! Pushed into a marriage of convenience, she’s caught between two men—one owns her heart, the other is bent on stealing it.
Percival Avery is a member of Nelson's Tea, an elite group assigned to protect England's shores at any cost. On a mission to avenge his sister’s death, Percy infiltrates the gang of cutthroats responsible. When his vessel attacks a merchantman, Percy must choose between vengeance and saving the life of his commander's niece. His only choice is to mutiny, but mutiny obliterates his well-laid plans. Forced on a new course that leads straight back to Constance Danbury, Percy has to make a decision—chase revenge or allow himself to love again.