A most interesting question

The concept of the "water around Torpedo" (WRT) tank was just for the purposes described by Don.

One of objectives behind WRT was to maintain the same weight distribution after launching the torpedo so that the bow (or stern) would not suddenly rise and break the surface. So launching a torpedo would mean replacing the same weight (as close as can be accomplished) in water so there is no change in the overall weight.

Would it not be possible to blow the WRT to sea and get rid of the ballast water after launching the torpedo? I don't know if the WRT could be blown to sea at that depth. Would use up a lot of High Pressure Air thats for sure (assuming that the WRT is even connected to the High Pressure Air system.

I do believe that Don's observation about the strength of the inner door would be a primary limiter though.