I think you should possess easily detectable training platform which would be easy to track. That is why I looked closely at declassified U-boat noise levels and other interesting data found here:
http://www.armscontrol.ru/subs/snf/snf03221.htm#table2
As you can see WW2 U-boats were very noisy beasts. I scaled above data to DW database passive SL values and I think that hypothetical U-boat in DW should have passive SL equals at least 74 dB if not higher.
So you could "transmogrify" one of AI subs (Foxtrot, Ming) in DW to noisy U-boat using DWEdit and try to hunt her with impunity all the time. :rotfl: