We were lucky enough to be here on a day when admission was free, but I believe this is actually part of the Military Museum on the island.
This is an older and smaller submarine than others we have seen. It held about 20 crew members and was used to patrol the Gulf of Finland from 1933 to 1944.
Finnish history during World War II is complicated. The Soviet Union invaded it in 1940, but the Finns bravely held off for several months despite being vastly outnumbered. This submarine actually sunk a Soviet Merchant ship. It was retired as a result of a cease fire and post WWII politics, but then converted into this museum some time later.
Entrance is easy through the side. The space inside is tight, but we walked through without difficulty. We thought that this visit was extremely worthwhile. We really came away with a feel for what conditions would have been like on board. Space was very tight even for a crew of about 20. They have photos of the crew practically covered in diesel fuel, which probably would have been the norm.
Suomenlinna Island is full of natural beauty, attractive historic architecture, and unusual sights such as this submarine. You likely won't stay long here, but you'll gain an appreciation of what life must have been like in a submarine.