I'm listening to the Little Mermaid episode of Myths and Legends and boy, is that a terrible story sending a terrible message to girls. Hans Christian Anderson, what's you problem?!
@Julia disclaimer: this is totally speculation. I am talking out of my ass, may have dates wrong, and have not had a chance to do research.
I suspect though that The little Mermaid is supposed to be a better take on the Undine novella.
Keyword though: "supposed to".
@Julia it's pretty much every fairytale, though.