Doctor Who: The Underwater Menace

The TARDIS arrives on an extinct volcanic island. Before long, the travellers are captured and taken into the depths of the Earth, where they find a hidden civilization - the lost city of Atlantis.

The Atlanteans worship a goddess named Amdo and use Fish People - men and women operated upon so that they can breathe under the sea - to farm the plankton-based food on which they survive.