Suisei no Gargantia

In the distant future, a young lieutenant named Ledo and his mobile weapon are caught in a space-time distortion during a battle with the alien Hidiaazu. Ledo awakens from hibernation on Earth, a planet submerged in water where humans live on massive fleets of ships. He lands on a fleet called Gargantia and meets Amy, a 15-year-old girl who works as a messenger. Struggling to adapt to a life without war, Ledo learns about the fleet's way of life as they scavenge the ocean floor for resources.