Here is picture of general convergence zones situation from my SoundPropagation simulator (
http://roger.questions.cz/other/SoPro.1.9.zip).
You can see that CZ goes deep, but not straight down. Note that this image is not much correct since it stops simulating the rays as they leave the rectangle and with convergence zones situation they would bend back and get 'into the picture' again.
Also this is raytracer, it applies only for high frequencies, so let's say it's applicable for active sonar.
While DW still can surprise with it's sound propagation model, it's much much simpler then this.
