Director Kevin Connor said "Doug McClure was a great asset. In fight scenes he was especially good due to his hours of American TV action films. He knew exactly where the camera was at all times and threw punches precisely where the effect would work for the screen. He was always co-operative and came up with many ideas."
The real U-33 of World War One served in the Mediterranean and survived the war.
Amicus cast Stuart Whitman in the lead. AIP refused to finance the film if Whitman starred, so he was fired.
Some poster art for this film contains images that don't appear in the movie at all. These elements include men wearing diving suits underwater, a diving bell with two passengers visible in a view port and some kind of manta-like creature attacking the submarine.
Announced at the time as being the most expensive production Amicus had undertaken up to that point. Though the film was a co-production.