OM is the answer! It started off with Monsun boats but has long since moved to encompass the whole of the U-boat war.
http://www.subsim.com/radioroom/showthread.php?t=134922
Be sure to get the Environment and Graphics Upgrade for it, too:
http://www.subsim.com/radioroom/showthread.php?t=158764
Looks great, don't it?
Check out the whole ATO mod forum in general.
I generally prefer OM to SHIII+GWX lately. There's only a few features I miss, and SH3 commander - on the whole, OM is super-solid and really well-featured. Up there with the best mods for any game.
Check out my screenies of my ATO campaigns on this page:
http://www.subsim.com/radioroom/show...08977&page=202
Enjoy the trip. I've also come back from SH5 for another spin of OM just now.