Gargantia on the Verdurous Planet

Far future. Humanity has abandoned the doomed planet and set its sights on the stars.
Lieutenant Red and his combat robot Chamber are soldiers of the Human Alliance army. Their task, as part of the fleet, is to protect humanity from the hostile alien race "Hidasu," a war with which has been waged since forgotten times.
Within the existing system, people are viewed by the Alliance merely as replaceable resources, each of whom performs assigned functions.
Red is no exception. He has spent his entire life fighting the Hidasu in his combat humanoid robot, and he could not imagine any other life until one day, during yet another battle, he fell into a temporal rift. Awakening, he finds himself in an unfamiliar place, his connection to the Alliance lost, and he has no idea how to return home.
However, an amazing discovery awaits him. The planet Earth, which everyone thought was dead, turned out to be habitable. Yes, almost its entire surface is covered by ocean, but people have learned to build huge ship‑cities from ancient relics that they lift from the sea depths.
This world is completely opposite to the one in which Red was born and raised. And during his stay on Garantia, namely the ship‑city where Red and Chamber ended up, he will have many amazing discoveries in this new world.