Currently closed but hopefully open for tours again at some point in the future, Type IX German U-Boat, U534, is the only sub I've ever been inside, but I have to say it was a unique experience in more ways than one, given that U534 was sunk and subsequently raised from the sea bed many years later.
This is not something that can generally happen, but since it was not classified as a war grave, it was okay to be salvaged. U534 is actually in remarkably good shape inside, with many personal items and original spares still in place. And yes, it is very cramped inside.
If you're interested in U-Boats, this site is well worth a look:
http://www.bignotion.co.uk/~kemble/uboat534.html