Gargantia on the Verdurous Planet (2013)

In the distant future, humanity has evolved into two distinct species: the Galactic Alliance, who reside in giant space colonies, and the Hideauze, mysterious creatures that live in space. Ledo, a soldier for the Alliance, ends up stranded on Earth, a planet completely covered in water. He is taken in by a fleet of ship-dwellers called Gargantia who scavenge the remains of civilization beneath the sea. As Ledo adapts to his new life, he becomes entangled in the struggles of the Gargantia crew and begins to question his own beliefs about war and humanity.