I would say get some paint on your brush, then brush a little off on a paper towel or something, then gently brush it on the blue you are wanting to do for waves. I'd use a stiffer bristled brush for that...as for seafoam, same idea with the same stiff brush, but instead of gently sweeping it onto the wave, do a technique called stippling. Get some white (and maybe even some blue) and tap the tips of the bristles on the brush on a paper towel to remove the excess, then tap it on the top of the wave. Should work out pretty well, I would think...but you might want to practice on paper or something first to make sure you like it.