GINGA NO UO: URSA MINOR BLUE

Urey is a boy who lives in a laboratory in a lake with an old man. One day they find a strange star in the Little Dipper. The stars are disappearing and the Little Dipper changes into a fish shape. To save stars, they begin to sail across the Milky Way to the place where the strange fish lives. (Source: AnimeNfo) — Description from AniList community (anilist.co).