Man From Atlantis: The Disappearances (1977)

Elizabeth is kidnapped, one of dozens of scientists held prisoner on the island of Felicitos, controlled by special mineral springs that render their victims completely happy and compliant. Dr. Smith is using them to build a rocket to take her away from our troubled planet in search of some better world, and Mark must find a way to counter the brain-washing and free the captives.

Unfortunately the movie Man From Atlantis: The Disappearances is not yet available on HBO Max.