HiTech told me a number of years ago that he'd be adding subs, eventually. One possibility he discussed was to have a "wolf pack" that would move like a fleet, and from which the subs would spawn (like PTs from the surface fleets). As for catching a CVBG on the open sea, this would be nearly impossible if we had big oceans and people operated the fleets in the same way as they did in real life. Neither is typical in-game. Remember that CVBG's spawn from fixed, known locations (i.e. ports). Add to that that our maps often have maritime chock points that fleets must transit. Finally, our fleets usually spend an in-ordinate time cruising back and forth off shore as the attack a particular base. Adding subs would likely alter that practice overnight, but still people would be unable to resist bringing a fleet close in shore to attempt to overwhelm a base.
As noted, adding subs would involve more than simply modeling a sub. It is a major undertaking, but one that even now may be on-going. Code can be put in and tested discretely, without us even knowing about it. Phase one, for instance, may be modeling the subsurface ocean terrain. That in itself isn't really that hard, and is probably within the capability of the terrain model. It may just not be visible to us, or available in the TE released to the public.
