This Enchanted Island. Teacup Magic, #6

Miss Metis Seabourne flees scandal and humiliation for a Continental sea voyage. and quite happily gets herself kidnapped by privateers. On board the Caliban, Metis finds a new family of misfit, piratical rogues including the handsome Captain Bones and his dazzling first mate, Calypso. She ditches the teagowns for britches, and learns to feel capable rather than ladylike. For the first time in her life, she is free to be herself.
Her old world, of house parties and magical croquet, returns with a jolt when the crew of the Caliban join a caper with a runaway queen and the mysterious Isle of Dream with its mermaids, sea witches and whimsical wild magic. Caught between two people she loves, Metis must learn to be braver than she's ever been before.