Well DW had very very weak effects until version 1.03 came out. Than it chenged to this:
http://www.subsim.com/radioroom/showthread.php?t=117814
In short - under the layer the signal is somewhat weaker, and under the layer at some range there is a shadow zone where you can't be heard (or pinged) at all. This is for surface to submerged and vice versa.
I didn't measure fully submerged vs. submerged yet.
I can't tell about SC .. and I have never seen such effects on DEMON .. but I simply wasn't looking for them.