Compound word (noun): submarine, sub.
The related verb to sukellus is sukeltaa, which means ”to dive.”
The most famous submarine in Finland is probably Sukellusvene Vesikko which is now a museum on Susisaari island at Suomenlinna sea fortress in Helsinki. Vesikko (the Finnish name for the ”European mink”) was launched in 1933 in Turku. It was given to the military museum in 1959, and opened to the public in Helsinki 1973 after restoration.