What kind of function are you looking for in your Mods, Graphical, Gameplay, Difficulty? Realism?
There are mods that can have an effect on all pf these gave a look at the catagaries here and look for some you like the look at.
http://www.subsim.com/radioroom/down...hp?do=cat&id=2
As you dont yet have the U-Boat Missions you may want to start from the back of the mods Page and work your way from there to the newer section as most of the newer posts and updates will be in Ver. 1.5
That said are you using Ver. 1.3 *What you get if you buy SH4 From Steam* or 1.4?