This is coolbert:
That recent Swedish submarine hysteria the result of this naval vessel?
"Russian Triton-NN SDV"
"The Triton-NN submersible boat has been the subject of a lot of speculation in Sweden over the last couple of days. I do not believe that it is in active service and thus would not be the culprit in the ongoing submarine scare"
"Submersible boats are a subcategory of Swimmer Delivery Vehicles (SDVs) in the broadest sense. The basic concept is to travel as far as possible as a regular boat and then use its ability to run awash (partially submerged) and fully submerged as you get closer to enemy radars. In general submersible boats are more versatile than regular SDVs"
A whole bunch of nations now have their own version of the SDV. U.S., South Korea, Sweden and presumably Russia what we know of. Others as well exist [?] but are not so well publicized.
Boat on the surface but can submerge for a period of time too. And is a "wet" vehicle! Interior is flooded when submerged, all occupants on SCUBA for that period.