I'm trying out making water surfaces from the clear plastic clamshell or separate top containers that some products, mostly prepared food, come in. These are the largest boxes and you can make decent-sized waterways out of them. Cut out the shape you need, and color the underside with some pale green or blue or whatever, then glue the piece into place in the area you're modelling. You can get the sides to fit seamlessly into their waterway basin or channel, even scenick the grass/underbrush on the banks in place before adding "water".
With the coloring on the underside, the top is shiny and flat like still water. And since it's semi-transparent, you can add "underwater" details which can be seen through it.
This technique works well for flat, still water. To add ripples and waves means applying some sort of medium to the surface, but I haven't had to do that.
