When guest star William Shatner appears on a viewscreen, the ID code at the bottom reads "JTK NCC1701". This stands for "James T. Kirk", Shatner's character on Star Trek (1966) and the registry number of the U.S.S. Enterprise, Kirk's ship.
Roy Scheider disliked the direction of the show in the second season, gearing it towards more heavy science-fiction elements and requested to be released from his contract. NBC partially obliged, requiring him to appear in only three third season episodes.
At first, the show was intended to be a realistic portrayal of the near future. When the show suffered low ratings, however, it was allowed to go in a more fantasy-oriented direction that included stories about aliens, lost civilizations, and even the Roman god Neptune.
During the third season, several episodes were shown out of order. As a consequence, the character of Lt. Brody dies in one episode and then reappears in a later one. Similarly during the first season the episode in which Admiral Noyce is appointed UEO Secretary General aired after two episodes were broadcast with him already in this position.
Jonathan Brandis (Lucas Wolenczak) is the only actor to appear in all 57 episodes of the series. In second place are Don Franklin (Commander Jonathan Ford) and Ted Raimi (Lt. Timothy O'Neill) who appeared in every episode except for "And Everything Nice".
During the original broadcast of the first season, during the end credits, Bob Ballard of the Woods Hole Institute would appear and speak about an aspect of current undersea research, usually related to the content of that episode. Ballard was also the technical advisor for the show and the discoverer of the wrecks of Titanic, Bismarck and Yorktown and someone who really has been in the ocean's depths.
In the pilot the character Lucas Wolenczak played by Jonathan Brandis wears a Florida Marlins shirt with words World Champions 2010. This is a tribute to Roy Scheider who starred in the classic sequel to 2001 A Space Odyssey.