To make the lake I simply cut a shape from foam core and beveled the edge for a smooth tabletop to terrain transition.
A bead of caulk was run around the edge and smoothed out to form the bank and left to dry.
The water part was then painted. I used a mix of greens and blues, gently blending the colors to be lighter and greener on the edges and darker and bluer in the middle. The banks were painted a dark brown. Brush- on high gloss was applied in thin layers, letting each layer dry between applications. It caused the paint on the base to pull away a bit, leaving white flecks. I guess it kind of looks like whitecaps. The bank was simply coated in the usual mix of ground cover.
Here is also an attempt at some swampland. I painted a foam core base Hunter Green and applied a layer of the same gloss used on the lake over the green. I then glued down some leftover scatter from the bottom of my bag of Spanish Moss. It's crap, but it was quick and works in a pinch.