Factual error: In several episodes, Hogan's men communicate by radio with a British submarine, and the dialog hints that the sub is submerged at the time. During the WWII era, submarines could not communicate by radio without surfacing first. In most episodes one might argue that the sub could be running shallow with a mast up, which would perhaps be within the technical possibilities of the era, but in this episode, the sub is talking to Hogan's men while under attack by a destroyer. This pretty much rules out running at periscope depth, because ramming was regularly-used tactic for killing subs that were in the process of diving or surfacing. To avoid confusion: Nowadays, subs can communicate while running several hundred feet deep by using VLF and ELF. However, these are definitely not capable of transmitting voice, but are text-only.
Factual error: In this episode, the Royal Navy submarine that acts as a radio relay for Hogan's men is hunted by a destroyer. Like in most such Hollywood scenes, depth charges are seen exploding right next to the submarine, which just shrugs the blasts off. In reality, any depth charge that went off closer than 100 meters was instantly deadly to a submerged sub.