Just had my first chance to try a night surface attack in a while - patrolling off the coast of Portugal in early November 1941 and found a C2 traveling with a small freighter on a moonless night, with clear sky and dead calm seas
Angling in from ahead/starboard quarter at about 7 knots (kleine fahrt voraus), I managed to get to within about 1500m and was just about to launch my torps when I got the word from my bridge watch that we'd been spotted - I rushed my shot and managed to miss
, and then the stern gun on the C2 opened up on me so I dove to 'scope depth and fired off one more torp - that hit the C2 under the stern and she quickly went dead in the water and settled by the stern enough to the point where her deck gun became useless. Therefore, I surfaced and gunned down the small freighter (which was unarmed), and circled back to finish off the C2.
So, it looks like you can get close enough for a night surface attack if you're lucky and the weather/lighting conditions are favorable - I might have been able to get a little closer perhaps if there had been a little heavier sea state to mask my bow spray/wake, though I'm not sure if the game models that or not. I know I've gotten spotted further out before when there was a significant moon out (half or more), so it seems that the game may well model the lighting effect at least.