The Mermaid Did It is the 4th book in the Murder by the Sea series by Carol Ann Ross.

A trip to Weeki Wachee in Florida proves to be very enlightening as Carrie and Don are led to this Mermaid Capital of the world, a resort in Florida founded in the late 1940s. Who would have thought mermaids existed? Aren't they fantasy? Oh, they are alive and well and are doing just fine in Florida, a few of them have even made their way to Topsail. But there seems to be different kinds of mermaids, not all are sweet, little Disney characters. Some are like those in Greek mythology, bent on destruction.

Carrie wrestles with how anyone can be so manipulative, so vain, so cruel. Someone with a conscience could never be that way. A cooler Don offers explanations Carrie can't accept. Mirror, mirror on the wall. Who's the prettiest mermaid of all?

She takes you down, Oh, how she takes you down.
Oh, how she makes you want to scream.
This momma from hell,
This mermaid, Estelle.