Writer/Director Pen Tennyson was in the RNVR (Royal Navy Volunteer Reserve) and was granted an exemption from service while he made this film. To gain some experience of convoy life he served on HMS Valourous on convoy duty.
There are shots in the film of real destroyers, including HMS Imogen (D44), HMS Isis (D87) and HMS Griffin (H31).
Judy Campbell was forced to play all her scenes with Clive Brook in carpet slippers, in order to reduce their relative height.
There was no cruiser by the name of HMS Apollo in the Royal Navy during the Second World War. There was an Abdiel-class Fast Cruiser Minelayer of that name, but it was not launched until 1943 and not in service until 1944. There was also no vessel of any sort named HMS Aquila, serving with the Royal Navy during the war.
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.