The password that Hardt is to use for his contact is the opening line of Heine's poem Die Lorelei, in which a beautiful woman, who lures sailors on the rocks, is used as a symbol of homesickness. When his crew joke about him spouting love poetry to a woman in the dark, they miss half of the point.
There was a U29 in the German Navy during the Great War. It was sunk in the Pentland Firth in 1915, by being rammed by HMS Dreadnought.
This was the first film to pair Michael Powell and Emeric Pressburger who went on to become an important partnership in British film history.
Although passed by the British Board of Film Censors on 10 March 1939 (with a "U" certificate) and reviewed in the Monthly Film Bulletin in March 1939, the film did not reach the cinemas until 3 August 1939. It opened at the Odeon, Leicester Square, and was doing quite well till its abrupt closure on 2 September 1939. Unfortunately, WW2 was declared the British government closed all cinemas for safety reasons.
St Magnus (the name of the steamer sailing from Longhope for St Romness and Thurso during the closing stages of the film) is the Patron Saint of Orkney. There have been several ships in that area with the name St Magnus.