The Maiden's Courage by Marry Lynne Gibbs
The best man on the pirate ship is a girl named Alex. Alexandra “Alex” Gardner is the reluctant cabin boy on The Bloody Maiden, a ruthless pirate ship run by the charmingly evil Captain Montgomery. The crew is convinced she’s a boy, and she hopes it stays that way until she has the chance to avenge the deaths of her mother and brother at the hands of the crew. All goes well until the ship takes a handsome captive. Could her feelings for him ruin her charade?