I have only seen the pilot for this series, but I thought that it was surprisingly good. Certainly better and more interesting than some of its contemporaries, like "The Six Million Dollar Man" and "The Incredible Hulk." The plot wasn't perfect, but it was nuanced and interesting, and the scenes filmed underwater were believable. There was a sparing use of special effects, and those that were used were decent enough by 70's standards. I don't know how long a show could survive where the hero's only gimmick is that he could breathe underwater, but the pilot's plot didn't feel forced or contrived. It was a good movie, in and of itself. I am forced to wonder, though, with the popularity of Marvel properties at the time: "The Incredible Hulk," "Spider-Man" and things like the "Dr. Strange" TV movie, if this wasn't originally an effort to bring the Sub Mariner to television. He bears a striking similarity to Patrick Duffy's character: both have amnesia, are from Atlantis, and are looking for their home.
