Harry Buttle
You might want to try Timetraveller's Crush-Depth util. It will change crush-depths for any sub.
The util includes 'suggestions' as to how deep particular subs could dive, although some seem a bit much.
As Beery mentioned, VIIC/41 and VIIC/42 share the same config file as the VIIC. Just something to keep in mind if you try this utility.
Look for 'SH3 Crush Depth V2 Program' at:
http://www.delraydepot.com/tt/sh3.htm
As mentioned about crew alerting 'diving too deep' warnings, the util doesn't fix this either, but you will be able to dive deeper. The random-depth setting is is kind of neat too.