Well... i think its a little different in Kiawe's case. He already got the Firium Z before Ash arrived in Alola. Ash has Rowlet as well, and as it seems now, Mallow will be the one who will showcase the Grassium Z (Since i now kinda expect every classmate will get a Z-Crystal for the typing they are specialized in). I think its kinda likely Ash will not get a Water type. This episode can also be an indication both Rowlet and Litten will fully evolve, so they can showcase their respective Z-Moves with Ash and with that Ash bascially doesn't need the Firium and Grassium Z (Since Team Rocket has Darkinium Z, Nanu can hand out a Incinium Z, while Acerola can hand out Decidium Z).
Actually, I do think Ash will obtain the Grassium-Z for a couple of reasons.
First off, SM36 mentions Ash uses Rowlet against Olivia. Despite Rowlet's type advantage against her, I thought this was an odd decision, especially since Rowlet itself is weak to Rock types. Pikachu has Iron Tail, has more experience, and is not weak to Rock types, so it would have made more sense than Rowlet. But what if Ash is using Rowlet because he earns the Grassium-Z in SM35?
Then it makes a little more sense.
Second, because I just don't see an opportunity for Mallow to obtain a Z-Ring in SM35, and I'm not getting the same warning bells that I got for SM32. Unlike that episode, where focusing on Lana instead of Mallow (who gives you the Stoutland ride in the games) or Lillie (her fear) made no sense unless she was going to get something out of it, it's natural that Mallow would get focused on for her own Trial.
..Yea but what does that have to do with anything? There's only one Water Trial, and Lana already did it technically, and I doubt they'll ever go back to it ever again.
If he gets a Water type like Dewpider or Wimpod, he's definitely going to use the Bug Z-Crystal with them. The Waterium-Z ship has sailed.
Because your argument was that Incineroar has its own Z-Move, so there's no issue with Ash catching Litten despite Kiawe having a Firium-Z.
Well, right back at you. Primarina has its own Z-Move, and assuming Popplio fully evolves and Lana obtains the Primarium-Z, she'd likely use that over the Waterium-Z anyway. So that shouldn't stop Ash from obtaining a Water Type.
It's also possible USM introduces some new Alolan form that's a Water Type, as well.