Hope Indy mentions between 1939-1940 , during “Phoney War” the war at sea was anything but Phoney. While German Navy attacked Allied shipping , Royal Navy too put up an effective blockade and cut German overseas trade and commerce , put Germany a difficult economic situation AND Royal Navy vessels engaged and transhed German counterparts wherever thy could. Look at a S Class Royal Navy submarine HMS Salmon on patrol :
On 4 December 1939, while on patrol in the North Sea, Salmon torpedoed and sank German submarine U-36 (which was on its way to Kola peninsula in north where Russians set up a German naval base according to Soviet-German Non Aggression Pact .) This was the first time an Allied submarine sunk an Axis submarine in the war.
On 12 December 1939, HMS Salmon sighted the German liner SS Bremen. While challenging Bremen, an escorting Dornier Do 18 seaplane forced Salmon to dive. After diving the Salmon’s commander, Lieutenant Commander E. O. Bickford, decided not to torpedo the liner because he believed she was not a legal target. Bickford’s decision not to fire on Bremen likely delayed the start of unrestricted submarine warfare in the war.
On 13 December 1939, HMS Salmon sighted a fleet of German warships in North Sea. She fired a spread of torpedoes which damaged two German cruisers (one was German cruiser Leipzig, the other, her younger sister ship, German cruiser Nürnberg. Leipzig was so badly damaged it never left Baltic Sea ever again). HMS Salmon evaded the fleet’s destroyers, which hunted her for two hours. Then next day another British submarine HMS Ursula torpedoed and sunk German escort destroyer F9 which was escorting damaged German cruisers.