This is an interesting topic. Some years ago on TV I saw a documentary about submarines and it appears that after the war the Royal Navy did some deep trials using old submarines to see if the estimated crush depth was realistic. Well, if I remember correctly, the calculations were more or less right. They submerged a boat on cables and kept on going until the pressure hull gave way. They then brought the boat up and examined it. Here is a picture from Google showing one of the results;

[images.imagestate.com]

The result was complete devastation and no one would stand a chance of surviving. Of course the estimated crush depth was based on all the fittings not failing and only the pressure hull giving way. I may be a little hazy in my recollections but I understand externally framed boats (such as the type IX) could withstand deeper pressure than internally framed boats (such as the type VII) but I think there are more knowledgeable people out there who can confirm this.