The sea painting in this demo was done with acrylics, on a canvas sized 46 x 122cm (18x48"), using a 5cm (2") brush. I chose a limited palette consisting of titanium white, raw umber, Prussian blue, and turquoise. While there are plenty of suitable sea color paints to choose from, these are my favorites (particularly Prussian blue, which is transparent when used for glazing and quite dark straight from the tube).
I started by painting in the cloudy sky, working wet-on-wet. Although I didn't sketch out a composition on the canvas, I did measure the canvas so that the sky would cover the top third of the canvas (see: Composition Class: Rule of Thirds).
Once I'd finished painting the sky, I let it dry before starting on the coastal hills which were to recede into the distance on the horizon. Once again I didn't sketch out the hills on the painting because I had a very strong image in my mind of how I wanted to do them and didn't feel a need. The hills are painted in a gray mixed from the raw umber, Prussian blue, and titanium white, with the proportions, varied to produce a variety of tones.
The little bits of blue you can see in the foreground are where I minimally marked the direction for the intended shore rocks using some left-over blue on my brush from painting the sky. The two dark splodges on the bottom end of the canvas are where I wet it front and back to get two small dents out.
As I painted the more distant hills, I lightened the tone and increased the proportion of blue in the gray paint mix, according to the rules of aerial perspective. I painted the bottom edge of the cliffs a little way below where I intended to paint the horizon. This way I definitely wouldn't have a gap between the top of the sea and the bottom of the hills that I would then have to "fill in" later.
Once the hills were finished, I started painting the rocks in the foreground. The rocks are painted with the same colors as the hills, but with considerably less white in the mix.
The rocks in the foreground were positioned to lead the eye into the painting, into the surf, and towards the horizon. (Note how the middle one in particular changes between the top and bottom photos.) They were painted slightly bigger than I ultimately wanted them to be so that I could paint the sea spray and foam over them, not simply up to them.
When I'd finished the rocks, I brushed the rest off the paint off my brush onto the canvas in areas where rock might show through the water. As the brush got drier, the marks got scratchier and rougher, perfect for the glimpses of rock you see through shallow sea water where there's a lot of foam.
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?.)
Once the whole sea area had been covered with Prussian blue, I started working into this with titanium white. If you compare the top and bottom photos, you can see how working wet-on-wet enabled me to blend the white and blue.
With acrylics drying as quickly as they do, blending necessitates working very quickly. This suits my personal working style, but if you need a longer working time, then you can add either a retarder medium to acrylic paint or use a brand that dries relatively slowly (such as M.Graham).
Look at the base of the cliffs and you'll see that I've painted the waves crashing up against them. The piece of coastline that inspired this painting has large waves crashing in, so there's a lot of foam visible to quite a distance. If you're painting an identifiable stretch of coast, this is the type of detail you'll need to obscure accurately for your painting to seem authentic.
I then turned my attention to the breaking waves, foam, and froth around the rocks in the foreground. This was painted by loading the brush with straight-from-the-tube paint, then dab the brush down tip first, rather than brushing from side to side.
From here until I declared the painting finished, I was tweaking -- getting the foam on the foreground rocks done to my satisfaction, creating a sense of waves in the open sea.
You can see that the hint of rocks under the water created earlier on gradually disappears beneath the foam. But having them there, even if only a little shows in the eventual painting, adds to the level of detail in the painting, adds something extra to draw in a viewer albeit at a subtle level.
This is the final seascape painting. The bottom two photos are details from the painting showing the degree of looseness that was used in the painting.
Once I'd declared the painting finished, I put it on display in my studio where I could see it easily. I always leave a 'new' painting out like this then, after a few days, decide whether it was truly finished or needed something more. In the meantime, I started another seascape, a similar scene but with mistier conditions.