This technique is the least predictable, but produces a very realistic result. We are going to work wet in wet, laying down the blue water first and then dropping in the windmill.
Have your paper lying flat for this technique. Lay down a wash of cerulean blue over the whole water area, and then wait for a little until this begins to dry. If you go in too soon with other colours they will spread to far and fade to nothing, and if you go in too late the paint may cause cauliflowers and backruns to form, or just not blend at all.
My advice is to test it out by dropping in tiny amounts of 'windmill' paint and see what happens. If it spreads out just a bit, then that's the right time to drop in the rest of the picture. Just touch in the windmill and allow the wet-in-wet effect to do the rest. Risky, but effective!