A uboat did sink a passenger liner, The Luisitania, but i think it was after 1939. :hmm:
Heres a good read of Uboat Captains talking about how easy it was hunting in american waters during Operation "Paukenschlag," or "Drumbeat." As one captain put it, ERICH TOPP (Commander, U-552): Compared with the difficulties in the Atlantic war, it was a shooting of hares.
http://www.pbs.org/wgbh/nova/transcr...12lostsub.html