Haven't played it, but appears to be a nice mod to use a jap sub.
http://www.subsim.com/radioroom/show...137761&page=53
If you have SH4 gold with Uboat additions, you can use a Uboat. There are several mods to go with uboats. Check out the Uboat additions forum.
http://www.subsim.com/radioroom/forumdisplay.php?f=232