Here is the most common, the foam has collapsed and formed a blanket over the water. Its surface is broken into holes by the movement of the water. Many seascape painters have spent a lot of effort analyzing this effect. Here is a close up of the middle of the above picture.
The holes in the water are separated from and decorated by those stringy lines and thin bands of foam. Painting these holes will reveal the form of the water beneath and establish perspective and movement in your foreground. It is also hard to do well. E. John Robinson, who's book I recommended uses a lot of these effects in his painting. Noted the perspected ovoids with the squared off sides that occur in the foam at this stage. There are also repeated lines in there. The "strings" and borders of the ovoid holes aren't random but trail backwards in the direction of the receding water. The foam around these holes is lumpy and takes light and shadow as its thickness varies.
As the foam recedes into the middle ground it is captured by the next rising swell and carried up its face which stretches those holes even more. Notice in the shot above how the water on the spill of this wave turns opaque as it becomes aerated on the way down. Then it hits the base of the wave and turns to foam. From the top to the bottom of that water there are three stages.
- The first, at the turn of the form is the color of the seawater and may have a dark edge at its top. Notice the slight "humpyness" that the top displays.
- The middle stage the falling water is beginning to aerate and goes white or gray as it becomes opaque.
- The last stage is the violent conversion to foam which bounces and dances on the surface of the undulating sea ahead of the incoming wave. The individual globs of this foam are streamlined back towards their origin. These gobs of foam are in clusters and increase in size from the point of their impact ( in this example to the right ). In some places the foam is more advanced towards the viewer and in some places it has fallen behind. There is usually a firm shadow beneath it.