Well firstly WW2 subs are just submersible pt's.  
Scenario A - fleet spotted heading towards airfield A, spawn your sub from airfield A in direction of fleet with normal N, S, E, W & H buttons.  Manuever into position, fleet approaches, kill it.  You may not want to do it, I would, other might.  Time frame short.
Scenario B - for dedicated players, spawn submarine, sail into enemy waters, sit off port, vulch CV, fun.  Timeframe med to long.
Additionally submarines were fitted with anti-aircraft guns, some very powerful and anything from 3in to 5in deck guns.  Some subs had 2 deck guns.  
But I'm not saying we should have subs, just saying they're viable in AH.