SOS Encounter: Melemele Sea (30%), Hau'oli City (30%, Only in Ultra Sun & Ultra Moon), Route 9 (15%)
Looking for Toxapex? It isn't found in the wild in this game — evolve Mareanie (Level 38).
Its base catch rate is 190 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 or Dive Ball.
Evolution: Mareanie → Toxapex (Level 38).
Mareanie in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)