Does anybody know of a coach that is giving lessons through a web cam. And do you think it can be effective?

I don't think they have the resolution to be of much use.

I'd agree about the resolution issue. For instance, when we do video analysis remotely, we are hoping for 30 fps and 600x400 size. When I do my own, I also shoot slo-mo 300 fps. Not something you can pull off with a webcam.Mostly they seem to be for what I'd call "talking head" video clips.