In October 1939, the German submarine U-47 slipped into Scapa Flow, a sheltered area off the shore of the Orkney Islands, and proceeded to sink the HMS Royal Oak, with the loss of over 800 lives.

Per one source:

The attack by U-47 on the Royal Oak was without doubt one of the most significant actions of the war, in that the threat posed by Prien and his colleagues finally received the complete attention of the British high command and the then First Lord of the Admiralty Winston Churchill. Churchill, who in an announcement made after the attack grudgingly described Prien's feat as "a remarkable exploit of professional skill and daring" [emphasis added] had always been aware of the "U-boat peril"; it was this change in general attitude that was to play a significant role in turning the tide against the U-boats. It was indeed fateful that only the Royal Oak had been moored at Scapa Flow that night; had the damage to the North Atlantic fleet been more significant, it is likely that the British campaign in the North Atlantic might have been given a severe setback.

Edit to add sources. Original source was Manchester, The Last Lion 2: Winston Spencer Churchill Alone, 1932-40,and the quotation is from http://www.u47.org/english/u47_sca.asp?page=5

The US Navy has named four ships, including a ballistic missile submarine, for the Indian leader Tecumseh of the Shawnee. He was most notable for uniting tribes against the US during the War of 1812. (Edit: War of 1812. --Tecumseh died in battle against the US in 1813.)

The captain of the destroyer HMS Glowworm Gerard Broadmead Roope earned a posthumous Victoria Cross for his valiant battle against the German cruiser Admiral Hipper on April 8, 1940.

Roope earned the VC partially because the captain of the Admiral Hipper Hellmuth Heye wrote to the British through the Red Cross extolling Roope's courage.

Source: Haarr, Geirr H. (2009). The German Invasion of Norway, April 1940., p. 96