Based on the novel "Event 1000" by David Lavalee. When the U.S. Navy practices submarine rescues it is designated as "Exercise 1000". If a real-life rescue is required it is designated as "Event 1000".
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.
The submarine used in a few "breaching" scenes is USS Trout 566. This vessel was one of the last diesel submarines. It was to be given to Iran, but the US refused to turn it over due to the Iranian hostage crisis. The scenes were filmed when the boat was active, with active naval personnel. The boat was decommissioned and scrapped, unlike its predecessor and namesake USS Trout 202 which is still on eternal patrol.