WOODLAND SCENICS has an awesome product called simply, "Realistic Water"; look for it in the railroading section of your hobby store. As for realistic muddy water, paint the base a mud color, put a thin coat of water over it, do a thinned wash "mud" paint over that, repeat thin coat of water, reapeat, etc--the key is to do it in layers. For shallow puddles, just a hole/depression in mud color with a thick coat oe two of gloss will suffice. I agree with IYYAYAS about epoxy; it's a valid way of making puddles, water, etc, but it's a bit "old school" with all the truly great and easier products out there today. Somehow I always wind up with my dog, frying pan and car keys stuck firmly to me when I try to use it!