There are so many different ways, all of which work and all of which have downsides and upsides. I used the Woodland Scenics stuff for my river: what is cool is that you can put it on it layers and stick stuff in it: look closely and you can see the "trout" I put in this fishing hole . . .
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I have two large "lakes" - 96 by 38 inches and 44 by 22 at the extremes, that are just plywood sealed and painted with a oil-based gloss (Rustoleum in cans, mixture of 1/2 blue, 1/2 green, with a smidge of brown) - three coats. It looks like smooth moutain lakes, etc., - the plywood is removeable - these are access hatches int he middle of the layout, etc.
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Another advantage is that I can run boats on the hard surface - the mechanism for the moving boat is about ten inches deep and right under the plywood.
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