Where to find Basculin in Sun, Moon, Ultra Sun & Ultra Moon

Only in Ultra Sun & Ultra Moon
SOS Encounter: Brooklet Hill, Malie Garden, Vast Poni Canyon
Super Rod: Brooklet Hill (25%), Malie Garden (25%), Vast Poni Canyon (25%)
Surf: Brooklet Hill (25%), Vast Poni Canyon (20%), Malie Garden (15%)

Basculin also appears in

How to get Basculin in Sun, Moon, Ultra Sun & Ultra Moon

Its base catch rate is 25 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball.

Basculin in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)