The nuclear-powered 'Hydronaut' was actually built as the 'Submanaut' in 1956. It was originally a working scientific submersible whose primary function was to inspect underwater pipelines, but was diesel-powered and carried only two crew members. It could also not circumnavigate the world. Later in the 1960s, it was used to shoot underwater films for various movie companies and TV productions in the clear waters off Florida and The Bahamas. Initially built by Martine's Diving Bells of San Diego California at a cost of $1.5 million, it was purchased in the early 1980s by Six Flags for $500,000 to be displayed at its Atlantis water park in Dania, FL and then by nearby Grand Prix Race-O-Rama for only $12,500 in October 1992. Sadly the Submanaut was cut up on-site and sold as scrap in the early 2000s.
The all-white ship that is seen at the beginning of the film is the USCGC Diligence (WMEC-616), a Reliance-class United States Coast Guard 210-foot Medium Endurance Cutter. She was commissioned in 1964, and began her service at the Key West Coast Guard Station (also seen in this film). The 'Dilly' is still in Active Service as of this writing (Mar 2020), and has lived up to her name receiving countless honors including numerous service commendations, medals, and special operations ribbons during her 55 plus years of service.
When Dr. Volker (David McCallum) is setting up the radio signal near the end of the movie the noise it emits is the same as the communicators from The Man from U.N.C.L.E, which also starred McCallum.
Brian Kelly was in the middle of his career starring in the Flipper (1964) television series, and Marshall Thompson was in his television series Daktari (1966). Both shows were also produced by Ivan Tors. Lloyd Bridges previously starred in the television series Sea Hunt (1958), which was also produced by Tors.