Can I just interject here and say that during WWII, Germany did experiment with launching V-1s and other types of rockets off the deck of a u-boat, both while submerged and surfaced. Now, I fully recognize that these experiments were a failure and not pursued (Mainly due to launch problems, horrible accuracy and inter-service rivalry). Time and greater experimentation can solve the launch problems, Hitler can just order it be done thus overcomming the rivalry between Raeder/Donitz and Goering, and in a pinch - If you only care about shock and effect - Accuracy can be ignored.
As this had already been tested during 1942 OTL, it is a viable option, although you would probably be lucky to get 3x rockets a sub.
Source is here. Ignore the V-2 part, I am not referring to that.