Lloyd Bridges decided to leave the show after four seasons because the producers wanted to emphasize cops-and-robbers plots, while he wanted to focus more on environmental themes.
When Lloyd Bridges complained about the weight of his twin air tanks during topside re-takes, the art director designed twin tanks made of balsa wood and had them painted silver. Only in the earliest episodes is Bridges using real tanks for above-surface scenes.
When the producer wanted Mike Nelson to wear a gray wetsuit, he had to have one specially ordered. Objecting to the high price, the producer bought a can of spray paint, sprayed it himself, and had two of the crew hold Lloyd Bridges (I)' arms up while the paint dried. When it did, Bridges couldn't put his arms down--the dried paint was too stiff. The producer wound up spending the extra money for a gray wetsuit anyway.
This was the first foreign show ever broadcast in Spain.
According to the Manchester/Reid biography of Winston Churchill, Churchill rarely watched television but did occasionally enjoy watching this show.