You do have a point, since he didn't catch any Grass-types.
Out of the possible Water-types in Alola, here's all the lines and which ones are out of contention.
Popplio line - Owned by Lana
Wishiwashi
Mareanie line - Owned by James
Dewpider line
Wimpod line
Pyukumuku
So basically, two lines are out of contention. Aside from Wimpod, I see Dewpider being the only other possibility for a Water-type Ash could catch. Pyukumuku can't attack directly which is unfitting with Ash's battling style, and Wishiwashi would be a Misty's Goldeen situation.