The endless motion and chaotic repetition of waves against the shore is hypnotic to watch but challenging to draw. If you are learning how to paint a crashing wave, begin by watching the surf below you while saying your observations out loud. Describe what you see in as much detail as you are able. You may notice that you occasionally stall out and become silent. This is a cue that your mind has wandered to something else. Bring it back gently to the surf and continue describing what you see. I will demonstrate some techniques that help me paint the surf. The object here is not to simply copy my demonstration but to play with these techniques and incorporate what is useful to you into your painting style.
I began by painting the shadow of the foam area. To do this I made an irregular purple-gray arc and let it dry completely. Now comes the big trick. I made rough irregular zones with a white wax crayon along the top and bottom edge of the foam area. If you tilt your paper as you hold it to a light you will be able to see where you have put down the crayon. You do not need to cover the whole surface of the foam area with wax, just the edges.
Now paint a graded wash below the wave that shades from light green to dark blue. This must be dark enough for the white part of the wave to stand out against it. The foam will appear white because of the contrast with the water. While the paint is still wet, paint an even darker swath along the base of the wave at the point where it begins to curl up. On the water that curls over the top, paint a graded wash that transitions from blue to light green. That wonderful rough edge of the foam is all created by the crayon.
If you are not pleased with the contrast between the wave and the foam, you can add extra washes to the wave surface and the background, slowly building up the darks until you like the effect. Once the washes have dried, you can paint streaks of surface foam on top of the dried paint with opaque white. I used permanent white gouache. Creating the effect of foam on water is a little tricky. For tips on foam, see my blog post: How to draw sea-foam.