I watched this sporadically when it was live on TV. It is set in the late 21st century, when Man has colonized (and found life on( Mars, has colonized the ocean floor and has an active space program and very advanced military. It is written very well in terms of character development, but the storylines got rather thin later in the series.
In many ways, this reminds me of Star Trek in the way it plays out from episode to episode. The problem with it is that we are still on earth. We do not get to invent new creatures and have hidden undersea civilizations. But they do it anyway.
Still this is worth watching for episodic fun in the vein of Star Trek TOS, Voyage Beneath the Sea, Lost In Space, etc. This is not a 1960s show, so it has some modern effects and a modern look.
William Shatner appears in Season 1, episode 18.