Gargantia on the Verdurous Planet

Set in the distant future, this story unfolds in a galaxy where the Human Galactic Alliance is battling a nightmarish race known as the "Hidiaazu." Young lieutenant Ledo, along with his high-tech mobile weapon, Chamber, gets caught in a space-time distortion during a fierce battle. When he wakes up, he finds himself on Earth, a waterlogged world where humans survive on massive fleets, salvaging treasures from the deep. Ledo lands on one such fleet called Gargantia, where he meets Amy, a 15-year-old messenger. Having only known a life of warfare, Ledo is astounded by this unexpected peaceful existence filled with new challenges and friendships.