The WW2 Finnish submarine
Vesikko, built in Germany, is preserved & open to the public at Suomenlinna, which is a fortress built on a series of islands off Helsinki. It was constructed by the Swedes in the 18th century; they called it Sveaborg & its defences were compared to those of Gibraltar. There are a number of museums, several but not all of them military & it's easily reached by boat from Helsinki. It's a nice place so a lot of the visitors are there for the scenery rather than the museums. There's also a permanent population & a Finnish Navy base.
Suomenlinna