All the submarines shown submerged at sea are models. They were shot "dry" in the studio. Smoke was used to supply the effect of sea water. 'The Los Angeles Times' article published on the 11th December 1976 reported that a model of a nuclear submarine was constructed on a movable platform and placed inside a water tank measuring twelve feet deep and forty-four feet wide on a sound-stage at Universal Studios.
Charlton Heston and producer Walter Mirish embarked onboard the USS Gurnard (SSN 662) to get one day of submarine service experience. The Gurnard was homeported in San Diego and transited early one morning to pick up the star and the producer in Long Beach. The Gurnard ran numerous drills that day which included flooding and fire. The Gurnard was captained by CDR Henry Goodman Childs who went on to serve as the first Navy Four Star Admiral as Commander in Chief, United States Strategic Command (USCINCSTRAT), from 1994 to 1996, the first naval officer to command all of the strategic nuclear forces of the United States.
Based on the novel "Event 1000" by David Lavallee. When the U.S. Navy practices a submarine rescue it is designated as an "Exercise 1000". If a real-life rescue is required it is designated as an "Event 1000".
The filming model used for the submarine "Neptune" is similar to that used for the "Tigerfish" for Destination: Zebra, station polaire (1968) - even down to the identification number "509" on its conning tower.