Future Boy Conan: The Big Giant Robot's Resurrection (1984)

Original title: 未来少年コナン 特別編 巨大機ギガントの復活

In the aftermath of a war fought by magnetic weapons which caused massive earthquakes, a young and spunky boy naned Conan gets involved in a grand adventure. Conan lives in a world that is mostly oceans; the major land masses having been destroyed by the weapons of the great war. He meets a young girl named Lana, and they eventually foil the plans of the city-state of Industria which wants to regain the lethal weapons.