I was just thinking about this recently, and I definitely believe that we should have U-Boats available. U-Boats played a powerful role in WWII, and it would be a mistake to omit them from this game.
Having said that, I think it would be important that U-Boats be restricted to being launched only from Ports, and not be available to launch out of just any VH from any VB or AirField. Also, we would either need to limit the number of U-Boats per country, just as we limit numbers of Carrier Groups (per Port), or we would need to place a rather high Perk Cost to each U-Boat.
It would be absurd if we could launch 45 U-Boats and surround an enemy Carrier Group and obliterate it in under a minute. Some semblance of reality must be retained.
The one (two?) U-Boat(s) per Port sounds like a good plan. I'm not sure what the best way to navigate the submarines would be, however. Given the slow speed, using the map-guided navigation system we currently use for the CV Groups would probably best. However, when it comes to spawning to the submarine, and initiating torpedo launches, it would be important to have the ability to do normal manuevering, just like we use with the current ground vehicles.
Adding U-Boats also adds in a little extra complication from the standpoint of defense. Would a submarine set off the proximity warning on a CV if it were near enough? How would an attack on a submarine work, if you can't steer the cruiser/destroyer independently of the rest of the CV group? Could we use the PT Boats to launch depth charges?
Lots of questions to answer, before it could be implemented well.
WP