Wadatsumi

In a post-apocalyptic world covered with water, Wadatsumi, a person who lives in what appears to be the last piece of land, finds a mysterious girl dragged by the waves of the sea. Where does she come from? Are there other lands in this world?

From Taruby:
After 'The Great Sinking,' civilization on Earth crumbled as the world was drowned in the ocean. One tiny band of survivors eke out a living atop the ruined remains of a skyscraper. Wadatsumi, a young tomboy, is saddled with the care of four elderly gentlemen, until a chance encounter with a group of pirates and a mysterious woman turn her world upside-down.