Most important thing to have (especially if you look at doing more than 1 copy of SH4 & modding them...

) id MultiSH4... so as to keep each mod set's game saves separate (especially NOT in an SH4 game save folder, either.

) as that would bollux up everything for them.
That said... moving on.
To add to that, on the Uboat side of it.. for SH4...
there is Niume's eng. translated work (ongoing...

) KSD II: Ace edition... as well, to round out the selection there.
On the U. S. side... to add to the list, is...
SubRon 50.
https://www.subsim.com/radioroom/sho...d.php?t=157750
This is factual, that there were some (bout... mhmm... 6, may have been 7... think 1 had to forgo the trip over due to mech issues, iirc...

) Gato Class Submarines that served in the Atlantic/Med area, for a short time... just note, that it is a short career... Sept 20th 1942 to November 1st 1943 (extra 3 months more than real life)
If you're interested... there IS... a Brit sub mod for SH4, as well.
Though this 1 requires a donation to get to... as it is in the bonus section of Subsim.
We dive at Dawn.
This 1, though... is not for the noob mod aficionado... as it DOES require some... tinkering about with... even I, am still working out the kinks in it... but, do like the look & sound of it thus far. Just have a few bugs... to iron (see kick) out of it.
Just to round out that list that propbeanie started...
M. M.