This Game isn't really setup for multiplayer...
These modern era subs are tasked with operating alone so there would never be a "Wolfpack" as in the Germans of WWII. Multiple modern era subs operating together, would just add to the chance of increased detection and ruin their solo, stealthy, independent footprint!
In order to really add multiplayer, VS each other, you would have to add playable Russian Subs and NATO warships... Would require a lot of work!
The only other way is to turn this game into a surface ship vs sub multiplayer game. I seriously doubt the Devs want this game to go in the direction of playable surface ships hunting OPFOR playable subs... But who knows?
I would prefer if the Devs kept concentrating on Bug fixes and Adding content at this point.
