Can't remember stock or other mods, but you get credit for any ship you sink you can't see with TMO as long as you hit it. Only thing that stinks is when ships explode and take out nearby ships, no credit. I've sunk four ships with one torp, hit one and the explosion took out three nearby. In reality you would get credit, not in game, but still fun to see how many you can take out.
Using the deckgun is easy in the storm, just use radar or sonar and get to about 500 yards from the ship, keep asking for bearings to nearest ship, look at the range and fire down the bearing. Once you set it on fire it's easy to shoot at.
I often use the sonar only method by RR, works great. However once they know you're around they usually start flasing lights, but you can see this light, giving you a bearing to shoot at with the deckgun or torps. Just get close, but not so close that an exploding ship takes you out.
Just be careful later war, many escorts have radar and capital ships with radar will direct those that don't to you. Nothing worse than having 3 escorts run right up your stern in a storm. If you run radar they will also home in on that, so I leave mine off except a few seconds to set up, but I change directions a lot to clear my baffles and ask sonar for nearest warships. Course I never run submerged in a storm.