The Dashing Sea Train and the City of Water: Water Seven!

あらすじ

After the Going Merry narrowly escapes being hit by a "sea train", the crew meets an old woman, Kokoro, and her granddaughter, Chimney. Kokoro gives Luffy a map to Water Seven. Luffy, Nami, and Usopp buy boats driven by sea monsters called "yagara".