It's a matter of preference for the scene/diorama builder. For a toy soldier dio, it's probably more effective to use a painted surface that has no real scale depth (like a real creek or river would). But if you're going for more detail-approaching scale modeling style-you'd probably want to use model railroad products (eg, Woodland Scenics) or other techniques that replicate water in depth.
But even for scale modelers, there are adherents to both techniques. I've seen ship builders who always use plaster for their bases, but paint the surface very convincingly.
I play around with both, depending on my mood.
Prost!
Brad