So, as we know - Joyboy made a promise to the fish men and broke it. Then left a message saying he was sorry that he couldn’t fulfill his promise and said that in 800 years he’ll complete it - So the Noah is not a battleship, but a transportation ship. Remember during Fishman Island that fish woman with the ability to see the future saw Luffy destroying Fishman Island? That is because Luffy and Shirahoshi will both transport the Fish men to the surface at the end of the series for one reason or another. The ship is also called Noah because of the biblical story of the Noah arc where animals were transported during the great storm.
