I just watched "Underwater Station". It's where the 3 missile scientist were abducted and they find one of scientist in an underwater bell. Mike NElson gives the guy his aqua lung and stays behind to mess with the kidnappers. He definately cuts the right hose. They goofed though and showed the diver with intact hoses in a short scene after that. Probably bad editing.
Considering they filmed all 150+episodes in about 3 years, I'm amazed they were able to pull off the stunts as well as they did.