Mermaids are often described as being composed of female torsos attached to fish tails, as in this scene of Daryl Hannah from the movie 'Splash'. This screenshot is copyrighted by those who own the copyright to the film.
SPLASH: Directed by Ron Howard. With Tom Hanks, Daryl Hannah, Eugene Levy, John Candy. A man is reunited with a mermaid who saves him from drowning as a boy and falls in love not knowing who/what she is.