Painting water is challenging. It's generally a good idea to focus on one of the three characteristics; movement, reflectivity, and transparency. Often we see more than one of these characteristics simultaneously, but I think it makes sense to play up just one, and subordinate the others because making a single statement almost always makes for a stronger painting.
Not saying you can't have more than one in your painting. Just saying emphasize one, and deemphasize the others, lest your impact be - ahem - watered down. (Sorry! too much wine!)
I'll just talk about the transparency today. There's really no trick to it. If you just paint the values and the colors that you see, you will have a pretty good depiction of transparent water. The problem is we have a hard time ignoring the thingness of things and seeing just colors and values. We tend to think, "oh that's a rock. so it must be this color." or "it's water, so I have to mix a blue".
If you can see color and value without thinking about what it is you're looking at somehow, that would be helpful. Easier said than done, right? You can learn to do this, but it does take a lot of practice. There are some things you can do to force the eye into seeing abstractly. Like looking at the view upside down by bending down and viewing through your legs. You'll look pretty silly but hey, who needs dignity when you're learning?
You can try not looking at the object directly, but use your peripheral vision to determine the color of the object. Or make your sight go out of focus. Take off your glasses if you wear them. You can also blink your eyes repeatedly like strobe lights. You can "scan" the view by moving your head from side to side and taking in the color but avoiding focusing on any one object. You can look through a small hole in a neutral colored piece of cardboard, isolating the color.
All these techniques are intended to disengage the mind from thinking about the thingness of the things you're intending to paint and just take in the color and value information.
But after you are able to see and determine the colors of all the things that need to go into the picture, you still have to paint them.
The way I approach it is fairly organized. I wouldn't call it a formula because it's too general to be one, but it's logical nonetheless. Like everything else I do, I try to establish a broad contextual relationship first. That means blocking it (the water) in, with an average color of the water (don't think water. Just think color and value), or a few colors that makes a simple gradation. In the paintings I'm showing on this post, the gradations might be made from a dark mossy green to an ochre-ish color. I try to keep it very simple - nothing more than a block in.
Also important is that all the other elements (trees, shoreline) are blocked in too. The major "areas" have to work together, especially value-wise, so it's crucial to establish these relationship at an early stage. In other words, I don't just work on the water and hope to do the rest of it later.
And then I look for submerged rocks and define them by indicating their shadows. What color are they? Well, since I approach my paintings tonally, the first thing I might do, is to just mix a darker version of the color of the water block-in. And then determine whether it's a little warmer or a little cooler, and make subtle adjustments by mixing a darker red or a darker blue, and draw them in.
Often I see that some shadows look warmer than others, so I make sure they're not all the same. Also, I don't copy the rocks as I see them. I take cues from what's there, but I don't hesitate to change shapes, sizes, and locations so as to redesign my underwater rock garden.
I mean, I think it's nice to be able to paint them exactly as they are, but I think it's nicer to end up with a compelling composition.
Oh, and these darks are painted fairly thinly, with transparent or semi-transparent pigments (Trans. Oxide Red + Ultramarine, for example). Dark areas like these represent shadows, cracks, and holes where light is not reaching, so I want these areas to be quiet and subordinate to the lit areas surrounding them. In juxtaposing thicker, opaque paint against thin transparent paint, the thicker paint will stand out more, while the thinly painted areas recede, (not talking about distance here. I'm just talking about whether an area jumps out at the viewer, demanding attention, or not) so I want to take advantage of these characteristics and paint dark, quiet areas with thin, transparent colors.
Once the shadows are in, we can see the rocks. Now we can differentiate each rock from its surrounding color by painting it in a color/value that is a variation of the original block-in color. If it's sunny, these rocks may be lighter, but that also depends on the local color/value of the rocks themselves, so they may be darker. You just have to look and make that determination individually.
Some of the rocks may be sticking out of water, and these are often significantly lighter than its submerged parts. They make great accents or punctuations in the composition, so I place them carefully, trying different positions, quantity, sizes, and shapes. Mother Nature didn't put these accents in the water with our paintings in mind, so they don't necessarily make the best compositional devices if we paint them literally.
On these above-water rocks, especially if the water is moving, we see darker values where it's wet but not submerged, right where water's surface meets the rock. To make sure they look wet, we have to make careful value shifts in this area. Often this dark wet areas have sharp edges.
So if we break down the color/value variations on a single boulder, we have 1)dry lit areas, 2)wet above-water lit areas, 3)dry shadow areas, 4)wet above-water shadow areas, 5)submerged lit area, and 6)submerged shadow area. And that's not even counting halftones and planar shifts. Of course, we can simplify it as much or as little as we can get away with. The amount of small variations we put on each rock would depend on its context, and the artist has to make those decisions based on whether more or less information helps or hurts the painting.
There are often very bright, small, sharp highlights where the water's surface touches the rock, too, as small waves reflect the sun directly into our eye. These highlights are really effective, but if you overdo them, they look hokey, so use them sparingly.
That might be a good segue into talking about reflections, but that's another day's post. This post kinda got long winded already. If you have read this far, thanks for your patience!!