Staying with German subs, there is
Operation Monsun from lurker_hlb3 from years ago, which was updated by Fifi in
Dark Waters. Especially with OM, be sure of what you want to do with the mod before you begin the "assembly" process. The mod was built in several stages with several patches also, as well as an environmental mod. You most likely will end up with a mod list similar to SH3 or TWoS.
For the US fleetboats, Ducimus's old standard
Trigger Maru Overhauled is the most popular amongst those wanting "difficulty" in their playing. Bubblehead1980 recently released his update to the mod
TMO Update BH. There are many others, but my favorite is
Fall of the Rising Sun Ultimate, which is not as difficult as TMO, though it is not as "easy" as Stock (which I never did find "easy"). Lots more choices, most good...