Op Monsun is the mod you want if u-boats are your thing, but it's rather bug-ridden and the guy who wrote it seems to have bowed out of modding. having said that. I'm pretty much hooked on it - There's an environment upgrade that goes with it called OMEGU, or a stripped down version [OMPLOT] - Sailor Steve ported the Das Boot sound mod over - if you install it, bear in mind it takes about 20 mins in JSGME - your machine hasn't crashed
also there a few bits and bobs like german chart labels which help with the look of the thing
Never played SHIII so I couldn't say how it compares
Happy hunting anyway