SOS Encounter: Akala Outskirts (30%), Route 14 (30%), Route 15 (30%) + 20 more
Super Rod: Malie Garden (5%, Only in Ultra Sun & Ultra Moon)
Surf: Malie Garden (35%, Only in Ultra Sun & Ultra Moon)
Its base catch rate is 45 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 Dive Ball or Quick Ball.
Evolution: Magikarp → Gyarados (Level 20).
Gyarados in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)