There is a liquid called realistic water from a company woodland scenics which you can purchase at a hobby shop or online. Here is a link:
http://www.woodlandscenics.com/
Follow it and look for the above mentioned product.
It is relatively easy to use. You just pour and wait for it to dry. It is best used in small amounts to make streams, lakes, etc...the key is your scenery preparation. Since the liquid will dry clear its underneath which matters. Prepare well and let dry.
There is also EZ water by woodland scenics but I wouldn't recommend it as it is difficult to use in every way.
The only con's about Realistic water is it is kind of expensive at about $15 for 16 onces.
I've seen it at hobby shows like at Greenberg's for $11...you'll have to shop around for a good price.
If you're just making shallow puddles you might also try Future. It's a liquid floor wax which dries clear when applied...It's not good for deeper water as it will haze over when applied too thickly.
Well,
hope this helps,
Take care,
Doug