In a type VII/41 I have gone about 290 meters deep without taking any water several times. When in a career with type VIIB I stick to a rule that I never let it past 235 meters. Maybe it could take a bit more but I don't want to lose my career for trying that.
I've gone down to 390 meters in stock SH3 with a type VIIC
. It was more like a bug but it didn't crush for quite a while even as deep as that. And if I remember right type XXI started taking water near 400 meters.