I think is more about a bad campaign design. Just getting out in the middle of nowhere, waiting for months to blow up a ship, is maybe realistic, but not fun. Go there, sink that, refill. So the whole campaign is about to find a large convoy, get into the right position, fire the torpedoes, escape then refill.
I was thinking how fun can be a more specific type of mission. Something that sound very easy and casual, but end with a huge blow. Something like this:
A ship loaded with important war documents/troops or whatever and with some light escorts will leave port X heading to port Y in the next few days. Intercept it and sunk it. Sounds damn boring, nothing special or hard.
But when we arrive there, we found out that Thomsen already sunk our target and the enemy escorts are hunting him down.

They managed to damage his ship pretty bad so he was forced to surface. So sudden the whole things go hot and we have to choose very fast .. help Thomsen or stay safe far away.
But things go worst. Out from nowhere a scout plane shows up and indicates to the escorts our position, so one of the escort ship is heading on full speed to us guided by the plane and his sonar. In the meantime Thomsen, our best friend, is screaming for help

Also your crew should start screaming, going crazy.. maybe some of the crew members start to disobey orders.. you know, like we saw in Das Boot. We have a FPS here, so I should be able to get out my gun and shot down any crew member who disobey me :P
Now this would be a very fun mission, we could feel the tension and every mission or campaign would be really dynamic. I mean, we would never know what we can expect out there and objectives are changing rapidly. You have to think, react and calculate very fast like a real captain. This game should be one of the most hardcore game ever, not some casual sim with nice graphics.