[sublynx]

Here's an excerpt from BdU's War Diary on the second of October, 1939 (taken from U-boat archive.net):

"A further order from Naval War Staff states:

Ships which, in the view of the C.O., are equipped to carry more than 120 passengers, are to be regarded as passenger ships. M.D.V. 87 (T.N.: German Naval official publication) explains this further. The main points to note are the number of boats - 4 or more each side of the ship, length and number of promenade decks, bull's eyes.

Boats were instructed on these orders.

The term "passenger ship" is a very wide one, and its final definition left to the C.O.'s impressions. It must be realized that in practice this order is open to very wide interpretation, especially as the submerged boat's powers of observation are limited."

So you're good to go if you wish to. I think the 2000 GRT passenger/cargo in GWX is a small passenger ship, but under 120 passengers. I don't think there was much civilian traffic during the war, but if you want to give time for their evacuation you could try to hit the ship in a way that would sink the ship slowly. (Avoid hitting the bow or the keel and not too deep.)