The Ring of Wind (Young Samurai, Book 7)

Jack Fletcher is battling the high seas. Ambushed by the Shogun's samurai, Jack and his friends have only one hope of escape—the Seto Sea. But with ferocious storms, man-eating sharks, and ninja pirates at every turn, their chosen route is fraught with danger. A treacherous crew only adds to their problems as they flee south from a ruthless samurai sea lord. Unless Jack can harness the Ring of Wind, he and his friends are destined for a watery grave.