>>52197858I had the same question when you posted this, so I might as well post what got me one in the last hour.
I ate a meal at the fish shop in the main city that gave me Water Encounter 1 and then went to a place I knew Tauros spawn. Besides the shitton of Marills there was a Water Tauros in the second group I checked. Easiest way to tell is to look at the tail.
Not sure what'll happen when I breed it with a Ditto but if it does work, then Dive Ball Water Tauros sounds pretty good for Surprise Trade fodder.