SOS Encounter: Route 7 (30%), Hano Beach (15%, Only in Ultra Sun & Ultra Moon)
Its base catch rate is 60 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball. For the look, the best colour match is Heal Ball, Luxury Ball or Level Ball.
Evolution: Staryu → Starmie (Water Stone).
Starmie in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)