That looks delicious. I haven't had watercress in ages, but I enjoyed it from what I recall. Incidentally I also just came across another recipe for a watercress dip which I want to try out as well.
This also kind of reminds me of a recipe Gordon Ramsay did with Broccoli Soup. He simply boiled the broccoli florets in heavily seasoned water. Then he thew them into a blender, reserving the cooking liquid. He blended them with some of the liquid and seasoned to taste then simply plated it and put some goat cheese in the middle. So insanely simple, but it looked fantastic.