Merman is a direct and memorable reference from CITW. Replacing it with a mermaid would be a disappointment to people familiar with the film.
Not knocking how badass mermaids are, don't get me wrong.
Don't worry, I thought it too.
"This house doesn't have gore. But what it does have is a very particular set of skills; skills it has acquired over a very long career of HHN's."