Harry Fowler's character wears a shirt that wardrobe have made up wrongly -- it buttons the female way, right over left. He wears it in most of the exterior shots and some of the interiors.
The U-53, which is shown arriving to effect the rescue of the Germans late in the War, had been sunk by the British destroyer HMS Gurkha west of the Orkney Islands in February 1940. The same fictionalised name was used for a late-WWII U-boat in another 1950s British film, The Cruel Sea (1953).