Sub vs sub warefare in WW2 was rare/nonexsistant. Especially in the early days without even the basic acousticly guided torps.
In ww2, most sub sinkings were either by depthcharge, or attacking the sub while it was on the surface running diesels.
In AH2, subs would be likewise vulnerable on the surface, and with the exception of the Type IXD with its large AA gun complement, or the 'ack-nest' moded type VII's, the subs would be quite unable to do more than dive.
If they added a sub, they would need to add depth bombs for aircraft at the least. And even then, the only subs I could see them adding are the early Type IIC (weakling), or the Type IXD series (fast enough to flank CVBG's...on the surface....barely)