First and foremost I'd recommend MekStark's
Reduced Underwater Visibility mod. Invaluable for immersion since Cold Waters' default setting seems way too clear.
Then I've made a couple of tweak mods, optional and perhaps due to become obsolete at some point, once the devs get around to balancing those areas of the game.
Try this if you find the campaigns too short to savour, and
this one if you feel enemy aircraft are too psychic when it comes to pinpointing the location of your sub.