One Piece Special: Open Upon the Great Sea! A Father's Huge, HUGE Dream!

Bonnie and Max, crew members on Pirate Zap's ship, are tired and want to escape, but unfortunately have no money. Three children held captive on the ship happen to overhear them. The eldest, Amanda, who's father was a pro treasure hunter, knows the whereabouts of a great treasure, and with that proposes a deal. They agree, and the five barely escape and make it onto a small island where they meet Luffy and his crew. Unfortunately they were pursued and now Luffy and the others must battle the Bayan pirates and find the treasure that Amanda's father had left for his children. Amanda, who has always resented her father and his love for adventure and treasure, finally understands his feelings.

Synopsis from Anime News Network