I've seen figures of 900 ft for Balao someplace. Not that they ever went that deep on purpose.
There are two issues. There is the depth at which even the toughest skipper would have started sweating bullets, and there is the depth at which the sub would actually fail.
The problem in the game is that we have too much information. Assuming the hull % is gone, then by messing a little with the total hitpoints, at least having any damage at all can randomize the crush depth and keep players honest
I suppose you could set it closer to what we know might be the real depth for a crush---possibly over 1000ft for a Balao. Then we add a new campaign layer. Random_crush.mis. The contents? A pacific-wide minefield with mines scattered from 600 to 1200 feet deep
There be dragons!
tater