Two of the U-boats named, U40 and U42, had been sunk in 1939 by mines and the Royal Navy (HMS Imogen and HMS Ilex) respectively. U37 (the one sunk in the film) fought through the war until 1945, sinking 55 ships and damaging 1.
Writer/Director Pen Tennyson was in the RNVR (Royal Navy Volunteer Reserve) and was granted an exemption to service while he made this film. To gain some experience of convoy life he served on HMS Valourous on convoy duty.
The U37 Commander says to take the submarine down to 50 feet. Being German, he should have said 15 meters, as Germany used the metric system. The Officers on the Deutschland, later in the movie, use metric measurements.
When the order is given to 'Cease Fire', an officer speaks into the speaking tubes. However, these were put out of action earlier with orders being passed along a chain of men.
When the aircraft approaches the Deutschland, a magazine can be seen on the observer's machine gun. When they decide to attack, the observer is seen to attach a magazine to the gun that now does not have one.