Johnny Whitaker (spelled 'Johnnie' in the credits) so impressed Brian Keith that when TV's Family Affair began casting later that same year, Keith requested Whitaker test for the part of his nephew.
According to a Los Angeles Times story, the fake submarine had been previously used in the 1965 Marlon Brando movie Morituri.
Although the action in the film is supposed to take place on fictional "Gloucester Island" off the coast of New England, most of the outdoor scenes were filmed in Mendocino California. Mendocino in the 1960s was a somewhat remote artist colony on a rocky cape projecting into the Pacific Ocean, about 100 miles north of San Francisco. The harbor scenes were filmed in NOYO Harbor, just south of Fort Bragg, where Carine's Fish Grotto and Cappy's Bar still exist to this day. (2006)
The Air Force fighters sent to bomb the sub are McDonnell Douglas F-101B Voodoos from the 437th Fighter-Interceptor Squadron at Oxnard AFB, California (now Camarillo Airport).
The name of the Russian submarine, converted letter by letter from Cyrillic, translates literally to SPRUT, which is pronounced "sproot." It means "octopus" in Russian.
The Russians have problems pronouncing the island's name, Gloucester. However, the Cyrillic alphabet used on the Russian maps phonetically spells "GLOSTER".
Unable to borrow a real submarine from the US Navy, and unable to bring a Soviet submarine to the United States, the art department built the Russian sub. It is powered by four motors underneath the hull, each driving a section of the "sub", and if you watch carefully, you can see it flexing where the four sections are joined.