SH4 is still a great game, the more so after receiving supermod treatment
If you are playing Uboats, then your mod of choice is no other than Lurker's "Operation Monsoon":
http://www.subsim.com/radioroom/showthread.php?t=134922
with its corresponding environment & interface mod:
http://www.subsim.com/radioroom/showthread.php?t=158764
If you are more into american fleet boats, then you can choose between:
-Real Fleet Boat:
http://www.subsim.com/radioroom/showthread.php?t=159709
-Trigger Maru (Harder but very exciting and with loads of goodies):
http://www.subsim.com/radioroom/show...t=trigger+maru
- Fall of Rising Sun:
http://www.subsim.com/radioroom/showthread.php?t=147988
Good hunting