Listen to chilipie.
I've shot U/W video (commercially), and am also a certified scuba instructor. Here are my $.02: A soft bag is only safe for work at/near the surface. It may be certified to 20m, this doesn't mean the camera is. As Cinematographers, our camera is a precision instrument and if you are taking it deeper than 3 ft, a proper housing is the only safe solution.
I'd never expose any quality equipment to 20m in a soft bag. The camera body & lens would be directly exposed to 3 BAR (that's 3X more pressure than sea level). Also, all your controls may become useless because some/all of them might be pressed down (at the same time) due to the pressure from the bag, the focus ring also may not turn freely.
There is a danger that while at depth air will migrate into small parts of the camera & electronics. When you surface this air expands and if it cannot escape easily, part of the camera, or a lens element may be forced to shift to allow the air to pass. Of course, you can see the danger here. A Good Example: A fellow instructor/friend had a small cavity he was unaware of. As he ascended, he experienced severe tooth pain that was only relieved when the filling burst out with a loud squeak and a 'pop' (as he heard it).
A rigid housing keeps the camera at sea level pressure (no matter what depth it's at), so the camera can function as expected.
Thomas