I'm going to mirror Dave's discussion here as it is a good question.
There are many ways to do it and there has been a lot printed on it too. I do suggest a little research and experimentation on your part to see with which method you are most comfortable. I have done it three ways.
1: Paint the plywood base with acrylic paints to look like water, dark blue to black in the middle and lighter as you approach the banks which are then painted earth tones. "Plant" (glue) rocks, old tires, stumps, WS reeds and grass and whatever you feel you want in the water. Then use Envirotex Lite resin to cover it all. Pour it in per the instructions. You can make rough water by brushing the surface before it sets to pick up little wave crests. Plant your boats before it sets too. This is a simplification of my preferred method.
2: Do everything above, but use the WS Realistic Water instead. The old WS water pellets that you melted in a tin can were very yellow when set. I'm glad they came up with the new formula. The WS Water Effects are great for falls and rapids and other places where the surface is not level.
3: Use a fluorescent light ceiling fixture acrylic diffuser, paint it blue, and call it a lake. You can't see through it, but it is good for distant background use. You can plant weeds and rocks on top.
4: A bonus method. I have not done this but have seen water made by just using acrylic paints built up over the background. Start with colors and finish up with clear sealer to give the surface a wet look.
It will be interesting to see what shows up in the other group. My best advice as always is to read, experiment and do what you are comfortable with.
Anybody else have more water ideas?
Fearless Leader