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.
The real convoy HX 25 sailed Halifax on March 5-1940 and arrived Liverpool on the 20th 1940. It consisted of 25 merchant ships (4 of which were not present at the start of the convoy), one ocean escort and between 4 and 6 convoy escorts, depending on date. 3 ships became stragglers and one of those came under aerial attack; otherwise there were no attacks or losses to the convoy.
USS Keeling (call-sign: Greyhound) is a fictitious boat and call-sign, however the pennant number of DD548, (seen clearly after the 3 destroyers depart the convoy) was a canceled Fletcher class destroyer. The first Fletcher class destroyer was commissioned 4th June 1942.
On January 25th, 2019, Sony Pictures announced that they would be pushing the film's release date back for more than a year. Originally scheduled to open two months later on March 22nd, 2019, the film was re-scheduled for release on Apple TV+ on July 10th, 2020. Additionally, Sony Pictures' other upcoming film starring Tom Hanks, 邻里美好的一天 (2019), also had its release pushed back from October 18th, 2019 to November 22nd, 2019 that same day.
The U-boats are seen firing forward deck guns that are positioned closer to the bow indicating the class of U-Boat would be Type IX, however, the conning tower shape/design is that of a Type IA which had the deck gun mounted closer to the tower.
The movie shows the U-Boat passing close to the Greyhound to be under it's cannon and immune to fire, however, in reality when this naval tactic was performed, the Captain would order hard rudder to cause the boat to list thereby bringing the cannon down to bear. This is a centuries old practiced counter maneuver. (This maneuver is demonstrated in many an old pirate or naval battle movie)
Not only is this the second WW2 movie where Tom Hanks sees violent combat, but it also is the second WW2 movie where there is a climatic battle scene where Tom Hanks is rescued from potential death by an air strike via airplanes. The first movie is Saving Private Ryan, but sadly, it is revealed that his character in that movie dies from his wounds sustained in the climatic battle, whereas it is revealed that Captain Krause in Greyhound survives the war.