HiTech has stated several times that he would like to add subs to the game. I believe it is only a matter of time. Patience.
I'm sure they will eventually be a part of the game someday. The tricky part is adding all the possible anti-sub methods. Depth charges, per se. Would a person control a sub based on score like they would a task group? What would subs be able to do? Kill CVs? Could many people man the sub or just one captain who can shoot out all the torps?