When the Captain refers to "Huff-Duff," he is referring to high-frequency direction-finding. This was a key method of how Allied naval forces detected and tracked German U-Boats: the Kriegsmarine's U-boat fleet maintained a high amount of radio traffic with its shore command and with each other for their Wolfpack tactics against Allied shipping. Huff-Duff allowed Allied naval ships to detect those radio transmissions and determine the location of those transmitting ships to attack them.
Filming locations include the U.S.S. Kidd in Baton Rouge, LA and HMCS Montréal of the Royal Canadian Navy.
The real convoy HX 25 sailed from Halifax on March 5, 1940, and arrived Liverpool on March 20, 1940. It consisted of 25 merchant ships (4 of which were not present at the start of the convoy), one ocean escort, and 4 and 6 convoy escorts (depending on the date). These ships became stragglers and one of those came under aerial attack. Otherwise there were no attacks or losses to the convoy.
To prepare for the film, director Aaron Schneider put together a website of all Naval-related information in use at the time depicted in the film. He effectively created a database for the cast and crew.
One of the ships seen in the movie is a digital recreation of HMCS Sackville. The last remaining Canadian corvette, it is permanently docked in Halifax, Nova Scotia.