One of Cristóbal Rios' mermaid statuettes
A mermaid was a being that was half-humanoid and half-fish and lived under the sea. They were depicted in stories such as Hans Christian Andersen's The Little Mermaid.
In 2024, Adam Soong described the way his late wife swam "like a mermaid." (PIC: "Fly Me to the Moon")
In 2370, Melora Pazlar was unfamiliar with the term when Jadzia Dax compared Pazlar's dilemma with that of The Little Mermaid's titular character. (DS9: "Melora")
Mermaids lived on the planet Golos Prime in the Clevari system. Q, Junior suggested taking a dip with these mermaids, during a brief visit to their system in 2378. (VOY: "Q2")
The name of Cristóbal Rios' ship, La Sirena, was Spanish for "the mermaid". (PIC: "The End is the Beginning") In his quarters aboard the vessel, Rios kept several small statuettes of mermaids. (PIC: "Broken Pieces")
A deleted scene from ENT: "Fallen Hero" revealed that diving for tourists on Risa was guided by genetically-enhanced females, capable of breathing underwater. When Travis Mayweather revealed this to Malcolm Reed, he immediately thought "mermaids."