I'll tell you something weird. In SH3 you can use the switches to open all the doors, and I've done so on many ocassions. More than once I've only fired a couple of torpedoes then gone deep. The game doesn't care if the doors are open, but I do. So I'm deeper than 25 meters and start flipping switches to close the doors that were still open only to be told "Too deep to fire torpedoes, Herr Kaleun!"
So you not only can't open the doors if you're too deep, you can't close them again either!