When painting waves by observation rather than from imagination, look to see how much reflection there is on the wave. You may see reflection from both the sky and from the wave itself. Just how much will depend on local conditions, for instance how choppy the sea is or how cloudy the sky is.
The two photos above show very clearly how the blue from the sky is reflected on the surface of the water, and how the wave foam is reflected on the front of the wave. If you're wanting to paint realistic waves or seascapes, this is the kind of observed detail that will make the painting read 'right' to a viewer.
