"Max depth" is kind of a nebulous concept though. There's the "test depth" which is as deep as they're willing to test the submarine. There's the maximum operational depth, which is some lesser depth that keeps a safety margin over the test depth. Then there's the "crush depth", which is the depth at which the submarine will actually fail, and is hopefully substantially deeper than the test depth.
Been a while since I've watched "Das Boot", but IIRC, there's a scene in the movie where, initially on the surface, they have a taught string strung across the sub. Then, as they dive, deeper and deeper, the taught string relaxes giving a rather frightening visual insight into how much the hull of the boat is compressing.