I don't know what portions of the MK-9 SEAL Delivery Vehicle are still classified and which parts are not. Since they have MK-9's and photos of MK-9's on display at museums, it must be the interior of the boats, which still have classified portions - dealing with inertial navigation.
The MK-9 SDV is very small, carries a two man crew and is very uncomfortable to operate. It was a weapons platform to deliver two MK-46 torpedoes.
This answers John Coffey's question from this morning on whether or not you could arm a very small mini-sub with torpedoes.
There were still two MK-9 SDV platoons when I was at SDVT-One (a reservist at the time) in the early 1990's. I have heard that they retired the MK-9.