Welcome to SUBSIM
USMC1419
I have not played the U-Boat side much in 4 ( yet anyway )
but I am sure that someone here will have more to offer on that
For mods check out the Mod's Forum for both the U-Boat side and Fleetboats.
The Fleets because there are some mods that can apply to either theatre.
Edit: U-Boat Mods:
Here
Fleets:
Here
Good luck with it all
RDP