Perhaps this is too obvious to mention, but even if the weather is crappy, wait until it gets
really dark to attack on the surface. It makes a world of difference in '39-40. Then again, if visibility is so bad due to fog that you can't see even one ship until it's almost on top of you, it might be better to call it off. There are always more ships out there. And the weather will improve -- usually when you're halfway home!
Viel Gluck!