Gray Lady Down is a 1978 disaster film by Universal Studios which starred Charlton Heston, David Carradine, Stacy Keach, Ned Beatty, Ronny Cox, and featured the first film role for a young Christopher Reeve.
A small experimental submersible, Snark, is brought in to assist with the rescue. Snark is very capable, but run by a nonconformist navy misfit (David Carradine). The tiny submersible is the only hope for a rescue.
Ronny Cox plays Neptune's executive officer, scheduled to take over command from Charlton Heston, Ned Beatty plays the Snark's second crewman, and Christopher Reeve appears in his first film role in a bit part for the sharp-eyed viewer.
The movie re-used submarine special-effect footage from Ice Station Zebra, as well as the large-scale submarine model, with that film's submarine (Tigerfish) appearing as the USS Neptune.
The movie is adapted from the novel Event 1000 by David Lavallee (1971).