Steeltrap,
We set the depth to 20 metres mainly due to the
Crash Dive Blues!.
The crash dive depth can be changed by editing the value CrashDepth=20;meters in the file NSS_UboatName.cfg in C:\Program Files\Ubisoft\SilentHunterIII\data\Submarine\NSS_U boatName.
The crash dive is slighlty faster than a normal dive command.
Sadly the crash dive blues will be with us forever.