Suisei no Gargantia

In the distant future, at the farthest reaches of the galaxy, the Human Galactic Alliance wages a relentless war for survival against a grotesque alien race known as the Hidiaazu. During a fierce battle, young lieutenant Redo and his humanoid mobile weapon, Chamber, are engulfed by a distortion in spacetime. When Redo awakens from his induced hibernation, he finds himself on Earth—a long-lost frontier planet now almost entirely submerged beneath endless oceans. Humanity endures here on vast fleets of towering ships, scouring the seafloor for salvage to sustain their fragile existence. Redo is taken in by one such fleet, named Gargantia, where he meets Amy, a fifteen-year-old messenger girl who helps him navigate this unfamiliar world. With no knowledge of Earth's history or customs—and having known only combat his entire life—Redo is gradually astonished by the quiet, peaceful days he never imagined could exist.