You hit the nail right on the head; too deep.
I try not to go much deeper than 200-210 meters. There's not much room beyond that before the hull begins to crush. Kinda scary the first time that happened to me - I couldn't get the boat to rise even at flank. Just kept sinking and sinking and *CRUNCH*.
I wonder though, would it help if one were to blow the ballast tanks? I didn't think to try that.