The underwater scenes were filmed at Weeki Wachee Springs theme park in Florida. The park, which opened in 1947, is famous for its live mermaid shows and is still operational.
Ann Blyth has no dialogue whatsoever in this film. She laughs, sings (without words) and cries, but never speaks.
In an article in "Look" magazine that came out at the time the film was released, Ann Blyth said that the hardest part of making the movie was trying to learn to swim whilst wearing the mermaid tail. She said that she practiced for more than a week before she felt comfortable with not being able to kick her legs to help her swim.
This film was shooting adjacent to the Deux nigauds contre Frankenstein (1948) soundstage. Tourists were shocked to see Glenn Strange's Frankenstein Monster having lunch with Ann Blyth in her fishtail costume. Both Strange and Lon Chaney Jr., in his Wolfman makeup, were invited to the 'Mr. Peabody' wrap party where they hammed it up as their film characters.