I'm not going to attempt a complete review of this film. It's far too human-focused and NOT PISCATORIAL ENOUGH. (It's a 1970 release directed by Dennis Muren and Jack Woods.)
I do, however, want to point out that the character in this story on which the entire story hinges is a college professor named WATERMAN. (Charmingly, he was played by sci-fi and horror writer Fritz Leiber.)
The kids who go to meet with the good doctor COME TO GRIEF, but not before they see the professor try to cross a stream, slip on a wet rock, and bafflingly DIE. They turn their backs on him for a minute, and when they go back to check on him again, he's DISAPPEARED.
I daresay it was unintentional, but I HARDLY NEED TO POINT OUT the similarity to the rather abrupt transitions our operatives used to make from Naked Ape to fish. It really takes me back, you know? The operative would get a little too close to the water and, BOINK, gone without a trace. The Shaved Monkey rescue services would JUST ABOUT GO CRAZY trying to recover the body, but like as not the "missing person" was right there in plain view, laughing at them in the form of a Minnow or Snapping Turtle.
And what's even funnier is the timing of this picture. The last time I saw people hiking through the woods in beehive hair and stirrup pants was just about the time we changed over to the newer method of changing over slowly.
IT'S ALMOST AS IF THEY KNEW WHAT WAS GOING ON!