There have been several attempts at this; unfortunately I could only find these three discussions:
http://www.subsim.com/radioroom/show...eal+navigation
http://www.subsim.com/radioroom/show...eal+navigation
http://www.subsim.com/radioroom/show...eal+navigation
There was a Real Navigation mod for SH3, but the problem there is that if you don't use waypoints the sub doesn't just drift off course, the helmsman doesn't even try to hold his course!
Don't know if SH4 is the same.