For sinking any cargo ship up to C3 with one torpedo a bow shot is the way to go, in my experience. (At least since I started using GWX - can't seem to recall this method being that effective before.) Hit somewhere within 5-15m from the bow, with impact set at 3-5 m depending on ship size and she'll eventually flood and go down. (In rough weather she'll usually sink quite fast as the bow is pressed deep under the waves, but even in relatively calm seas she'll eventually start pointing her nose downwards, and the beauty of it is that the ships own momentum will help sinking her.)
This removes the need for remembering where to aim and what to use - even the most useless of landlubbers can find the bow of a target
The only downside is - it's a very boring way of making the kill. Not the flash and panache a cocky Kaleun would go for....