There are as many ways to sketch water as there are bodies of water. Think of oceans, streams, still lakes, waterfalls, floods, even a rain-wet street. Just map the shapes, perhaps doing a small, fast thumbnail or an under-sketch guideline in pencil, and then jump right in! (Figuratively, of course.)
A quick sketch before you start may help to clarify what you’re seeing in your mind or simply slow things down and set the mood. Take a deep breath and give yourself time to really see what you’re looking at. Pay attention to the shapes of streams and rivers, as well as to the negative shapes that surround them like banks and curves. Accurate observation helps these waterways to look natural, and perspective comes into play once again.
Warren Ludwig has carefully observed the shape of the small body of water in this marsh.
Painting a waterfall, or any other moving water, isn’t that difficult. Just pay attention to repeated shapes, look for where the turbulence occurs and see if you can tell why. Leave white paper for sparkle, and vary the color to keep it lively. The medium you choose also affects how you depict moving water. An ink sketch will use a different approach, as would an opaque medium like gouache or acrylic.
You can scrape out light sparkles with a sharp blade once the watercolor wash is dry. Notice the broken white lines on either side of the falls. I keep a tiny Swiss Army knife in my kit that works perfectly for this.