Hehe, fog and rain conditions is the most nerve thriling experience I guess
Before the development of radar watch crew had to trust on their eyes only and believe me it had to be not the most pleaseant feeling see warship or merchant bow starting to appear from a fog straight at you
You should feel safer while in the fog but your mind keeps telling you to be even more cautious.
So in short poor visibility can help you sneak past through warships undetected (if you are not too loud) but also can end your career by surprise.