Let's look at a few of the details in the painting to get behind the art. First of all, as I noted yesterday, I wanted to convey the social nature of gulls and so painted several in the picture. You might notice that their interaction is much more random than ducks, which fly together and act more coherently as a group, or sandpipers, which fly in flocks with remarkable precision.
Taking a close-up look at the nearest gull we see a few points about shading. If you are shading, you need to remember that sometimes reflected light will lighten a shaded area. Even though the area under the gull's neck is in the shade, it is receiving reflected light from the water, so it is lighter than other shaded areas. Also this reflected light will have a bluish tint, as it reflects diffused blue from the water.
We get to see the interesting light gradients in the gull on the water too; it also creates reflections in the water in front of it, but these reflections are broken up by the ripples on the wave.
The lobster buoy creates reflections too. Again, note that the reflections occur on parts of the ripple that mirror between your eye and the colour in the buoy. By planning these reflections carefully, we can create the ripple of the water flowing past the buoy.
A final point about painting water which we can note from this painting. If you look at the background water (below), you will note that the contrast between the light and dark areas of the waves is much less in the background near the horizon than it is closer to the viewer. The change in contrast is not as obvious in photographs, but consciously increasing contrast as you come to the foreground creates perspective just as much as drawing perspective in the shape of things.
This change in contrast is sometimes called "atmospheric perspective". And because water doesn't have the defined shapes of buildings, roads or fences, we need to use atmospheric perspective to give a sense of depth to a sea painting.