Steel by Carrie Vaughn
Sixteen-year-old Jill, a competing fencer, finds a broken tip of a rapier on the beach where here family is vacationing. She picks it up and it magically transports her to an eighteenth-century pirate ship. The ship's captain, who happens to be a girl, is fighting with another pirate named Blane, whose sword is missing the piece Jill found. While Jill struggles to find her way back home, she is caught in the middle of the fight. What will it take for her to find the secret to the magic sword?