i actually hit somethings with a torpedo!!!! ... and died afterwards...
I was inbound to a fleet which was heading straight for me. I had to slow down the PT boat so I could see the line of the first torpedo in order to compensate. This made aiming a little easier, and for some reason, the fleet didnt' seem to fire on me until it was tool late.
+1 for this achievement as I know it's dang near impossible to land a sunken ship in a PT.