Lemax, who makes lots of small holiday village houses, etc, also make blue water mat for use in their displays. I picked one up cheap during Michael's after holiday sale, I think about 70% off, with plans to use it to make rivers & lakes out of.
So far, I've only done 1 test piece, but since someone on TMP asked about them, I figured I'd show what I've done so far. Here is the lake I made.
I spray painted some heavy plastic sheet a light blue color & then used spray adhesive to stick the mat down.
I decided to spray the plastic blue 1st because the mat directly over the white plastic seemed a bit too stark in contrast between the white & blue. Here's a picture of the mat over some white plastic.
Since the mat isn't flat against the plastic, the contrast between the blue & white might be hard to see. But hopefully you can get the idea compared to the other picture.
The blue mat over the light blue color seems to work well. For rivers, I might try to vary the amount of paint sprayed along the plastic to get more variation of the color that shows through.
I just need to figure out how to add some sort of green flock/edging around the lake. I'm guessing any basing material or flock won't stick well to the mat.