A reader recently asked me in my comments section about painting water, and as I am in the middle of painting Venice scenes I thought it might be good to "reflect" a bit on it here (pun intended). As we all know, pure water is transparent and has no color. It's power, pictorially speaking, lies in the colors and shapes it reflects. It's always a bit dangerous to apply too many formulas to painting, but some general guidelines are useful (just be sure to verify these with your observing eyes!)
Obviously, if you are painting a still body of water like a pond or lake that reflects the surrounding landscape, the reflected elements are upside down and reversed in the water. Reflected shapes are sometimes foreshortened, and water's movement also distorts the shapes reflected, depending on how much of a breeze or current is at play.
A common error is to paint reflected items tone for tone exactly as they appear in their solid counterparts. But unless they are in deep shadow (which does sometimes happen in the narrow canals of Venice) dark elements usually appear lighter in their reflections, and light tones appear darker. For me, painting the reflections (and especially the dark values) fairly thinly works best, as standing water has a glass-like appearance.
A common error of beginners is to paint everything reflected in horizontal strokes, and in doing so, overwork and over-blend these areas until everything is kind of a muddy mess.
I like to paint the basic value-shapes of the reflections in downward or vertical strokes first to follow the forms above, and then add strokes of movement horizontally. For detail and highlights, it's easy to "overdo" them in reflections, so take a subtle approach to start. Sometimes that is the most effective. You can always add more touches later, but it's harder to take away unless you just scrape down or wipe the whole thing clean!
Moving water like rivers, rapids and ocean waves are another thing altogether. They have their own unique properties, and probably could benefit from their own (future) post!
Some of my favorite reference sources for painting techniques regarding water (and everything else!) are the books by Emile Gruppe. Gruppe was a wonderful impressionist painter and teacher who was a part of the Cape Ann School of artists. He worked in and around Gloucester and Rockport Massachusetts. He wrote a triad of books on painting and they are all invaluable to the landscape painter.