To further add to the realism of scale model pools and water features you can add color to the textured surface of a flat styrene or plastic sheet. In this photo you can see how a thin coat of Pebeo Vitrea glass paint has been brushed onto sections of the textured plastic pool surface, creating the effect of dappled water. The areas where the water needs to be clear to enjoy the koi in the pool below have been given less color than the rippled water sections which will be near the waterfall. In this case a mixture of green and blue colors were used on the surface.
To make the most realistic effects, use light layers of color and check between applications to see how your water surface layer compliments the landscaping you did on your water feature. If you still need darker areas of water, you can add more color to the top surface to give it the effect of dark water beneath. You have the option of using glass 'stain' paints or markers on either side of your clear plastic water surface to give even more of a layered effect, just check that the paints or makers you use do not destroy or cloud your clear plastic. Test them on a scrap of your water surface materials before you apply them.