Future Boy Conan (Mirai shônen Konan) (1978)

After the great disaster of 2008, a war that destroyed the planet, the world is now largely ocean with the continents having sunk. Conan lives on a remote island with his grandpa and nature, never having seen another human being. But one day a mysterious girl, Lana, washes up on his beach. The two become quick friends, but she’s soon kidnapped and taken to Industria, a technological remainder from the world before. Conan leaves his island in pursuit, braving new lands and many hardships with new friends and enemies just beyond the horizon.