From the "reliable" Wikipedia for a Type VII:
Test depth: 230 m (750 ft) -Calculated crush depth: 250–295 m (820–968 ft)
Very similar to the Gato figures, but the American boat designers were more nervous about their advertised "test" depth. Notice that the German crush depth is also calculated, and is very similar to the US calculations. Only the US used 1/2 of what they knew was a "safe" depth as the test depth. Think about it though, you are in a boat that is "safe" to 700 feet, and someone is raining depth charges and maybe a hedge hog or squid volley or two at you at that depth, and I do believe that everyone of us would have to change our shorts after just the first nearby explosion... Most submarine officers were highly educated men, most of them engineers in one field or another, and all could guess what the designers knew...