Oh, you're very right. If not RN then USN. Merchant gunners were their own division, and if you look through the ship reports at
www.uboat.net you'll see many cases in which a u-boat attacked with its deck gun and then dove and ran when fired upon by the merchant. That was the norm, but there were exceptions. On a dark night the merchant as seen from below is a black outline against the sky. The u-boat as seen from above is, well, not seen at all. Suddenly the merchant is being shelled from out of nowhere. The gun crews have to get to their guns and prepare them. The loaders have to form the chain from the magazine to the guns. By that time they've taken several hits, and then they find they don't even have a target to shoot at except for the muzzle flashes in the dark, and when your eyes are looking at black on black a sudden flash is more likely to blind you than give you solid information on the target location.
Yes, a surprise attack at night can be devestating.