Simulators and multiplayer just don't go together well in my opinion. Time Compression would be a huge issue, especially with this game. Also, what can you really do that would be more enjoyable with other people out there? I guess if we could control the Destroyers, but this is a sub sim, not a target sim. Also, do you want to tool around thousands of miles of ocean, looking for the one other person on the server?
I tried multiplayer on Flight Simulator before. It wasn't that great. It was just like "hey... I can see your helicopter over there... uhh... what do you want to do now?"
Even co-op on the same boat could get boring. I mean... wouldn't everyone want to be the one on the scope, and firing torpedoes? I wouldn't want to just play as the weapons officer, entering TDC data and doing nothing else.