Hmmm, methinks you're confusing crush depth and test depth here. Engineers get specifications from a customer (in this case the USN) and in order to make certain it "meets or exceeds" the requirements, and continues to perform to minimum standards after time, use and abuse. Normal standard is 150%, in other words the specs call for a 2 ton jack, design it for 3 tons and no matter how old it gets it will never fail at 1,998 pounds.
Military contractors usually overengineer 200%, or two times the requirements, often more if it's possible without compromising something else - for example Army wants a tank that can survive a direct hit with a 100 pound bomb, design it to survive a 250 pound bomb and it will always, under all conditions, survive a 100 pound bomb.
Subs are the same way, design depth is what the specs call for - also called TEST DEPTH, the depth which the civilian contractors dive to with the Navy buyer on board to prove it meets the requirement, then the Navy will buy it. If it doesn't come back from the test dive USN says to the builder sorry about your luck, hope you got insurance, because it's your boat until we officially accept it.
CRUSH DEPTH (officially "COLLAPSE DEPTH") is purely theoretical, since nobody ever came back from an actual implosion to say "we got crushed to death at 567 feet". In fact the opposite, the BALAO had a test depth of 400 feet, estimated collapse (crush) depth was 600 feet, they routinely went to 600 in emergencies. Some instances they came back and reported accidents where they lost control and exceeded 800 feet without collapsing. The German type VII U-boat had a test depth of 230 meters, calculated crush depth around 280-300, also exceeded in several accounts.
So if you're looking for realism, whatever the real world test depth says you need to add at least 50% to that for the ingame crush depth. For an S class, since they were 20 years old when the war started, 300 feet is reasonable. For the fleet boats I double the specs, test depth for PORPOISE and SALMON/SARGO is 250 feet, crush depth is 500 feet, for the GATO 300 feet is 600, BALAO 400 feet is 800.
Main difference with the S class was not only the age and how many dive/surface cycles they went through (metal fatigue from squeezing and expanding the hull over and over is one reason for overengineering, still meets the specs after 10 years of abuse) but also the fact that like all subs before the fleet boats, the hulls were riveted rather than welded. The PORPOISE class was the first with an all welded pressure hull, no leaks from old rusty rivets working loose (or exploding like in Das Boot).
Little fleet boat trivia - PORPOISE was about 302 feet long, SALMON/SARGO 308 feet, GATO to BALAO 312 feet. Actual difference was in the length of the torpedo rooms, all fleet boats started with the same cylinder 16 feet in diameter and 250 feet long.
