[johnno74]

In real life sitting on the seabed doesn't make you invisible to sonar/ASDIC. Sonar works on the principle that when a sound wave hits something with a different density, part of the sound is reflected back. About the biggest density change possible is when the sound wave travels through a sub's pressure hull, then reaches the air trapped within.

So if you are sitting on the bottom in real life, yes you will be camoflagued to a certain degree because the sonar "pings" reflect off the denser sea floor, but a trained sonar operator can definitely pick up the much stronger reflection off your submarine. Now I don't know a hell of a lot about sonar besides the scientific principles, so I can't tell you just how easy it is to pick up a sub on the seafloor with WWII equipment, but it is certainly possible. With modern equipment it is trivial. Even a basic fish-finder would show up a submarine on the bottom as plain as daylight.

In WWII U-boats sitting on the sea-floor were NOT invisible.