Writing
The screenplay, which was adapted by Wendell Mayes, differs substantially from the original book. In the novel, the ship is British, but in the film, it is American. The screenplay's final scenes of mutual respect between the protagonists are not taken from the book. In the book, the destroyer captain takes a swing at the U-boat captain while they are in the lifeboat because the U-boat captain claims that the destroyer crewmen are his prisoners. The film also alludes to evil in man (as personified by a concept such as the "devil") being the real "enemy" ("You cut off one head and it grows another..."), the force within that drives one man against another, or even against himself.
The screenplay has historical precedence. On 6 May 1944, USS Buckley, which was the lead ship of the same destroyer escort class portrayed in The Enemy Below, actually rammed and sank a U-boat in combat before capturing many of the German crew.
Mayes preferred the original ending where the two captains died, after the American tried to rescue the German. "The point was Dick and I both felt that these two characters were men who had no reason to live," said Mayes. "There's much more feeling if they should die, one finally trying to help the other after killing him. But the studio said, "No, you like both of them. You can't kill them. It'll disappoint the audience."...One of those things one must do."