Suisei no Gargantia

In the distant future, a young lieutenant named Redo and his mobile weapon, Chamber, are pulled into a spatial distortion during a battle with the alien Hidiaazu. They awaken on Earth, a planet now mostly submerged and inhabited by humans living on massive fleets. Redo lands on the Gargantia fleet, where he meets Amy, a 15-year-old messenger. Struggling to adapt to a world without war, Redo learns about life on the fleet and the practice of salvaging underwater relics to survive. Unfamiliar with Earth's history and customs, he gradually experiences a life of peace, a stark contrast to his previous existence of constant combat.