Yes, it's quite tricky. For testing purposes, you can apply a simple dark blue shader to your surface and add a white surface layer upon that. Enable shaders in the 3D preview, and then play with the intersect underlying values. So you can see in the preview immediately, what these values do. I have to say, that I don't really understand, how this works, but trial and error gives you (hopefully) the desired effect eventually.
I don't know the dimensions of your plane, so I don't know, if it helps to use my settings for the white foam (see image below). My plane had a length and width of 150 units.