Song about a man who finds a mermaid and eats her?

This is really beginning to bother me. I know I read a book awhile ago the protagonist of which was a young girl, and there was one scene where she was listening to someone sing a song about a man and a mermaid. The person singing can't remember the last lyrics of the song, but the girl does and imparts to the readers that, in the end, the man suddenly kills and eats the mermaid. Since then I've always wanted to know what song it is and what all of the lyrics are, but I can't remember what book it was that contained this scene and google isn't forthcoming today. :/ Anyone else heard of the song, or know what book this is from?