Submarine Vesikko is a compact but memorable stop on Suomenlinna. The vessel served in the Finnish Navy during the wars of 1939-1944 and is the only surviving German Type II submarine, a prototype design that later shaped Germany's wartime U-boat fleet.
The interior is cramped, metal, and practical, which is exactly the point. Visit it as part of a Suomenlinna day rather than as a separate trip from the mainland, and expect a short, focused look at naval life in very tight quarters.