Gargantia on the Verdurous Planet

Ledo is a soldier from a space-faring civilization locked in endless war with alien creatures, and after a battle goes wrong he crash-lands on an unknown ocean-covered planet that turns out to be a long-forgotten Earth. Stranded and unable to leave, he must integrate into the peaceful seafaring community of Gargantia while his AI combat mech Chamber processes this unfamiliar world. The series is a thoughtful science fiction story about the meaning of purpose, humanity, and what it means to live rather than just survive.