Painting water poses an unique challenge to the artist. It is transparent and yet has it's own physical presence that can be very difficult to capture. Water reflects light in very unpredictable ways and its colors and patterns are hard to understand. The artist needs to observe very closely so that the artist can learn to simplify what they are observing.
Moving water can be a very difficult thing to record, but if you watch moving water in a stream or waterfall or any seascape for a time one will notice that the movement follows definite patterns, which is repeated again and again. Once one acknowledges and understands this, one can start to paint with confidence, letting one's brush strokes describe the movement.
Plein air landscape practices can be a powerful learning experience for artists of all levels in developing observational skills and helping the artist learn from multiple challenges presented to them.
Painting a flat expanse of water seems easy, but the problem for the inexperienced artist is that if often appears to be flowing uphill, but in reality it is a flat horizontal plane and must be shown as such. If your water is painted the same all over, it will look like a wall, so you will have to find a way of suggesting recession. Putting in a few ripples or loading floating matter in the foreground is one way of doing this and making ones brushstrokes smaller in the distance is another.
Waves, fast moving streams, and waterfalls are more difficult to represent than still lakes, streams, or calm seas. This is partly due to a failure to understand and observe the behavior of water and the patterns of its movement. The shapes made by water as it swirls around an obstruction, or waves as they swell, peak and curl over onto themselves follow certain patterns, so it is always important to sit and watch before you begin to paint.
Another problem is deciding how you are going to catch this feeling of ever-changing movement. As a general rule let your brushwork describe the flow of the water using directional strokes that simulate its movement and follow its sweeps and swirls.
Don't be afraid of the water and take the dip!!