IMHO as they say, I see WW2 sims as a time machine into the past. With immersion playing and graduating up to the most difficult settings to test yourself as much as a particular sim offers, one should read up and understand on the conditions of the period. I know a few history buffs here do! To those who would like to know more, here are links
http://sonic.net/%7Ebooks/u-boat.html
and of course a great website devoted to U-Boats:
http://www.uboat.net
There is nothing like matching knowledge and a quality, immersive simulation. I look forward to others posting their favorites!