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, but all the same I thought I ought to share this side profile.
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 (like the US Mk.VIII) because they have the range to operate without a mother-ship in many cases. They are also faster 9above the surface) and can generally carry more passengers. But they are also less stealthy and slower underwater so it is a compromise.
As you can see it is essentially a boat hull with a box-like upper section with access panels on the roof. Pop-out thrusters act both as underwater propulsion and steering.
Triton-NN is a Russian firm's attempt to re-enter the SDV market. It is up against stiff completion and is not thought to have sold any units. The main competitor in fact is the Swedish developed SEAL Carrier:
Another similar offering is the South Korean VOGO SDV-1000W:
Both the SEAL Carrier and SDV-1000W are actually based on SubSkimmer technology and incorporate a Rigid Inflatable Boat (RIB) configuration. Subskimmer are currently the market leaders but their products are more photo-shy!
another submersible boat which is the US made Stidd MRCC:
Lastly I should mention that North Korea and also Italy separately pioneered submersible boat 'SDVs'.