The tug used in this movie was H.M.S. Restive (HMT W 39), a Royal Navy Rescue Tug of the Assurance class. This class of tugs was built between 1940 and 1943. With a 1,350 horsepower engine, these tugs could make fourteen knots and were powerful enough to haul just about any capital ship back to port. Five (out of twenty-one built) were lost during the war. The ship numbers used in this movie (W 83, W 86 and W 88) were not used by any ship of this class during the war.
Two endings were shot according to tcm.com. One had David join Stella on the train, the other had him just miss it, but promising to find her. This was apparently to satisfy the Motion Picture Production Code since the characters were not married.
After just a few weeks of filming, Writer and Producer Carl Foreman wanted to replace Sophia Loren with Ingrid Bergman but William Holden persuaded Foreman to keep her.
Hairy chested William Holden was once again required to undergo a complete torso wax for his singular, abbreviated shirtless scene, but Kieron Moore was allowed to retain his natural foliage for his.
Almost all of the shots on-board the ship at sea were taken at a visible angle to the horizon. Most shots on land are rigidly horizontal with occasional uses of the angle for plot purposes in certain scenes.
A wall calendar establishes the time of the story as January 1941. Tuesdays and Fridays are in lighter type to represent Meatless Days, a World War II rationing provision. By the time this movie ends, the U.S. has entered the war, and Christmas 1941 is celebrated amidst the bombing.
Although the American Film Institute Catalogue of Feature Films 1951-1960 claims that Belita (listed in an H.R. Production Chart) is not in the finished movie, she is indeed in it, in a small but worthwhile uncredited role, with dialogue, dancing with William Holden at the canteen.