No effort whatsoever has been made into maintaining the illusion of Michelle Phillips' mermaid tail: when John Saxon picks Naya up at the beach, her knees can be seen bending inside the silicone tail. When she dives into the water, there is a gap between her back and her fish waist. (The "scales" of a mermaid would be attached to her skin.) There is even a close up on the tail's side zipper about 24:22 minutes in, when Phillips swims with Saxon underwater.
At 35:45, when the professor and the mermaid are kissing underwater, it can clearly be seen that it's stunt actors. Particularly, because John Saxon's stunt double has a full head of thick black hair, and the front part of it floats up and down, revealing no hair loss patches.
Michael Duval's poison ring has no powder pouring out of it.
If Mr. Roarke and Tattoo are on another Island to rescue Julie Brett from Michael Duvall, what is one of the Fantasy Island car's doing on it?