8. Set the brush size, hardness, opacity and fill according to taste. The size is very much dependent on the area you want to treat – for the web size version of your mill shot, I started with ~60 for the larger areas, then went smaller to deal with the areas of water bordering on the rocks.

Now “paint” on the water. Wherver you paint white, the desaturated layer will show through. Whever you don’t paint, the background shows through. The degree to which the layer shows is controlled by the opacity & hardess of the brush, and the opacity of the layer itself. You really have to experiment with different settings to see the effect. If you don’t see any effect, try setting the brush opacity to 100%, and paint pure white.