Here is the story of a US WW2 fleet sub which went down to 1,000 feet and survived.
http://www.geocities.com/jaob/deepdive.htm
The Hull was so damaged by the pressure that the sub was taken out of service. As a Balao class, it had a test depth of 400 feet.
And this is the result of a really deep dive, USS Thresher which during a deep dive in 1963, sank below crush depth and imploded from the pressure.
http://www.history.navy.mil/photos/s...t/ssn593-l.htm
i.e. door:
insulation:
