The underwater scenes of the Concord are not of a new nuclear sub; they appear to have been taken from Ice Station Zebra (1968).
This was one of four episodes from the original 1966-1973 series that were remade during the first season of the 1988-1990 revival series because of a writer's strike. The original (from season 4, episode 7, originally broadcast in November, 1969, and also entitled "Submarine") involved a Nazi war criminal who knew the location of millions intended to start a neo-Nazi coup. Because the Nazi plot was no long credible nearly 50 years after the end of World War II, the team's opponent was a criminal U.S. Navy admiral trying to peddle a computer virus to enemies of the U.S. The remake, which otherwise closely follows the original plotline, is credited solely to Dale Duguid, normally a visual effects artist, even though the 1969 version was scripted by Englishman Donald James.