From MangaHelpers:

This story, in the universe of Suisei no Gargantia, follows Bellows, a side character in the original series, where the planet is covered with water and humanity lives in various large fleets of ships. The story begins three years before Ledo (from a spacefaring civilization) arrives. When Bellow's fleet is destroyed, she is rescued by a fleet of salvage ships, which send teams of yumboros (human operated excavation robots) under the water to recover goods. Her skill with yumboros is valued by her new fleet, and she soon joins the salvaging team.