Welcome aboard supcapn!
We are always glad to have someone new to push around, err, I mean help share our vast wealth of knowledge and guide to success.
For the US side: TMO ups the difficulty level to "ouch." RFB renders the boat and ships to as realistic as the game allows. RSRDC changes the traffic to historical realism. Traveler's mod makes many different changes to add another dimension to the game play. OTC makes corrections to errors in the game play optics.
For the German side, Operation Monsoon is an Atlantic campaign for SH4.
All the supermods are excellent in their own right, it's just a matter of personal preference.
There are many smaller mods available for individual customization of the game.
Make sure you read the documentation before installing the mods. They need to be installed in a particular order, or it will mess with the game.
Don't forget JSGME. It's an excellent tool to help manage the mods.
Now go polish the torpedo tube doors. The outside doors, they look kind of rough.