The 1978 science-fiction TV show Man from Atlantis follows the adventures of Mark Harris, a man thought to be the last survivor of the lost city of Atlantis. He has the ability to breathe and talk underwater, and has webbed hands and feet that aid his swimming. That makes him almost a merman as it is, and in the episode "The Siren" he meets a siren that has an actual mermaid's tail.
The siren is played by Carol Miyaoka, and although we never see her see swim or talk human language, she understands English and her singing voice has a powerful narcotic effect in addition to being able to short circuit electronics. In this episode she had been in captivity for some time, and is being used by unscrupulous men to bring down boats for their nefarious purposes. Mark gets involved in their latest scheme, and along the way he helps save the siren so she can finally go home.