Check this thread:
http://www.subsim.com/radioroom/showthread.php?t=117814
I'm not sure if I tested it but I think you wont get CZ contact from under the layer. It should be possible and zones of focused sound should go deep. But it's quite complicated, and what more it depends on frequency a lot.
As for DW, I simply suggest you try it. My guess is they won't go under the layer.