Paint the blue of the sky. The sky should be a darker blue at the top and a lighter blue at the bottom, so the sky should gradually lighten as it approaches the horizon line. Use a wide, flat brush to paint long, horizontal strokes across the canvas. Start painting at the top and add white to the blue paint at even intervals until you reach the point on the canvas where the ocean will begin.
Dip your flat, wide paintbrush in dark blue paint and mix it with a tiny bit of green. This will be the base color of the ocean. Paint the ocean across the horizon in a long straight line, then fill in the color of the ocean from the horizon line to the bottom of the canvas.
Dip a smaller paintbrush in the dark blue-green paint you used for the ocean, then lighten that color slightly with white paint. Paint a series of slightly curved, horizontal strokes of paint over the base color of the ocean. Make larger strokes near the bottom of the canvas. The strokes should become smaller as they near the horizon line, with the smallest closest to the horizon. These strokes represent waves. The more there are and the closer together they are, the rougher the seas.
Dip your small paintbrush in white paint. Apply the white paint to the top of some of the waves on the ocean. This represents the crest of the waves and will only appear on some of the waves. (Choose which waves to add crests to at random.)
Dip your small paintbrush in white paint again. Paint a few white clouds in the sky. To paint puffy clouds, make large, voluminous white shapes with rounded edges against the blue of the sky.