Max surfaced speed of diesel-electric boats was on the order of 20 kts; submerged it was 8-10 kts, but that would leave the batteries flat in less than an hour. German boats incorporated a snorkel later in the war, to allow them to run submerged with diesels, but only in fairly calm seas. Eventually, the Allies developed airborne sea-search radar that could pick up the snorkel, at which point the Germans began putting some radar absorbing material on the snorkel to reduce its radar cross-section (move and counter-move). But I digress...
Subs would likely have PT-like spawn points, so there would be known areas of the map where you'd be risking your CVBG to bring it in close to shore. Also, HiTech talked about adding a moving spawn point to represent a wolfpack; it's course could be plotted and changed in the same manner as a surface fleet; you'd spawn a sub from that. I don't know what he had in mind for how you'd sink a wolfpack; perhaps there would be a sub-tender with escorts, though it seems this would be horribly vulnerable to air attack. In any case, the wolfpack would likely only move at 2/3 or even 1/2 the speed of a CVBG. It would be useful to move it into a position in a bottleneck, to pose a risk to transiting CVBGs. That and as point-defense of coastal bases is the only way you could hope to engage the CVBG. Of course, along with the subs, you'd have to add all the means of detecting and attacking them.
