Well, that came as the series was rebooted into a camp comedic show. In the early seasons they were going for something more like "Voyage To The Bottom Of The Sea."
For some reason, this thread brings out my inner Contrary Member. When Raquel recently passed away, I went reading a little about her biography. She told of her first acting job in "Fantastic Voyage." Long story short, she had one line in the entire movie and could not remember it. So they had to write it on the wall behind the actor she was addressing so she could read it while looking in his direction. She many have had many positive attributes, but "great actress" was not among them.
It's an interesting story. They initially were going for more emphasis on the science and less on the fiction. Jonathan Harris came along later and threw a wrench in the works on a set where there was already tension created by egos. While everyone else was fighting over who got top billing, his late arrival got him "Special Guest Star" status on every show, which pissed people off. Then he grew a cult following and everyone else was bland by comparison. I never really liked the show because the story lines were stupid. I believe the show died not from lack of viewership but from lack of scripts...they ran out of ideas.