I can understand your logic on him not catching any Alola Water-types since aside from Popplio (who is going to Lana) not many of them evolve. He didn't catch any Grass-types in Kalos since Chespin and Pumpkaboo/Gourgeist were owned by Clemont and Chespin respectively, while Gogoat was Alexa's, leaving only Phantump who wasn't owned by any main character and only had spotlight in one episode. My guess was the writers felt Phantump's Ghost-type doesn't suit Ash's team, which is odd because Ghost is one of the few types he still hasn't owned yet, alongside Steel, Psychic and Fairy.
Considering what you said about the huge number of non-evolving Pokemon in the Alola Pokemon lineup, this could be the first time he may possibly have more than one non-evolving Pokemon on his team at once, since he did have a few in the past (such as Heracross, Torkoal and currently Hawlucha).