Probably the greatest accomplishment of any U-Boat was by Gunther Prien who sunk the British Battleship Royal Oak in Scapa Flow which was the Home base of the British fleet.
The Finns sank a Russian freighter using a surplus German training sub during WWI. Treaty of Versailles banned them from having subs as they could threaten the Soviet Navy operating from Leningrad.
Finland always fought on the German side as they would have ended up like Poland ( inside the Iron Curtain ) if they were on Russia’s side.
Maybe - although I believe it was Archerfish that sank Shinano, a much bigger Japanese aircraft carrier, in Japan’ s inland sea.