nobody knows. The German VIIc\41 had a thicker pressure hull than most U-boats. Ive also heard of a balao class reaching 8 to 9 hundred feet and living to tell about it. But in the american navies the average test depth was 250 to 300 feet. In the U-boat it was 100 to 150 meters. Both American and German navies have out of necessity exceeded that figure by some margin and lived to tell about it.
