The biggest difficulty in playing the sort of multiplayer campaign that you (and, to be honest, most of us, as well) would like to do, is that MP is set at TC1. You can't accellerate the time any faster than real time. That's the major reason all MP games seem to be 'mindless hunting trips': they start with players' U-boats within a reasonable, playable distance from their targets.
It is possible to run an MP games for days, should someone choose to do so. However, there are quite a few drawbacks to that. One is that there are no 'in-game' saves. If the host, for some reason, loses connection, the whole enchilada is lost. Kaput. If any player loses connection, they are gone as well, with no way of returning.
There are also no in-game pauses. This means no way of pausing the game to hit the latrine, grab some grub, go to work . . . try and calm down irate XOs who are about to thwack your noggin with a rolling pin.
Granted, you
could submerge to 20 meters and cut your engines if you needed to take a break. Much better than staying on the surface and coming back only to discover you've been bombed by aircraft. But that's also not terribly 'realistic' either.
Assuming you've got very cooperative Kaluens (and spouses/girlfriends/etc.) and everyone had rock-solid connections, I could see setting up something that ran for 48 hours over a weekend. But anything longer than that (and that's be a stretch, too, without a lot of coffee or Red Bull) I'm afraid it's just not possible to have a long-running multiplayer game.