Trigger Maru Overhauled(TMO) will make your hair stand on end. It tends to make you act more like a US sub commander by the way it is designed. TMO is what I play but haven't looked at the latest version yet.
Real Fleet Boat(RFB) is very good too. It is more realism based.
I've tried both and both are excellent mods. It just depends on what your tastes are. Try both and see what you like.
RSRD is still around and is probably a must to go along with the above mods. Make sure you use the correct version for whatever mod you use.
And JSGME is a must have to make enabling mods a snap.
There's a ton of other mods that you can use to season your game to taste.