Just because the weather is lousy doesn't mean the war stops. Simply remain submerged and track your targets via hydrophone as already mentioned.
If its a single contact you are in luck, you can track it to triangulate its position and then get a workable firing solution without using your scope
I did that in my last patrol and it worked! Had I wanted to find the ship otherwise I never would have unless I accidentally rammed it.
I sunk a small merchant fired a TI at 900m he never knew what hit him. I was able to get close enough to spot the fire burning on his deck. I surfaced and sped towards him to see what the heck I shot at having no idea since it was litterally a shot in the dark.
Here is what I found through the fog
Just in time too because he was going down!
Hydrophone triangulation works people! There is thread on this forum showing exactly how to do it. I'll try to find the link and post it here.