Now that the background (hills and sky) and rocks in the foreground were in position, I started painting the sea, using Prussian blue to create a dark blue in the sea that will serve as a dark under-layer to the waves and foam that will be painted later.
If you compare the top and bottom photos, you'll see the range of tones that Prussian blue can produce, depending on whether you're using it thinly or thickly. In the foreground you can see how the rocks show through the glazed blue.
I applied the Prussian blue by brushing it on undiluted towards the horizon line, then working in down towards the foreground, adding a little water to thin it as I did so. (See Acrylic Painting FAQ: How Much Water and/or Medium Can You Add to Acrylic Paint?.)
