I play DiD, let the crew shoot the gun, always return to base (career ends if I can't make it back due to fuel or damage), dive on all air contacts. If I intend to transfer I sail to the base I will transfer to (but not close enough to get Dock at: option) I play with MCU enabled, all other settings checked (manual targeting, no externals, etc).
Passenger liners were not off limits to uboats
http://uboat.net/allies/merchants/ship/643.html
http://uboat.net/special/biggest.htm