The Australian animated television series Fairy Tale Police Department follows the adventures of FTPD officers Christine Anderson (heh) and Johnny Legend as they go around solving cases and making sure that fairy tales proceed the way they're supposed to in reaching the happy ending.
In the episode "The Fishy Tale of the Little Mermaid", Christine and Johnny are on a boat when Johnny falls overboard and derails the little mermaid's story when she falls in love with him instead of the prince. Wacky hijinks ensue - Johnny has to fend off the little mermaid's overenthusiastic pursuing of him (it is indeed a very different story when the little mermaid doesn't take no for an answer), and Christine has to deal with how she jumped into the sea witch's transformation spell and got a mermaid's tail. Along the way Christine uncovers the sea witch's true intentions of marrying the prince herself.
It's an all-out slapstic spoof, with some bonus shout-outs to the Disney version - the little mermaid is combing her hair with a fork at her introduction, she says her father's name is King Triton, and the sea witch's magic is contained inside a shell she wears around her neck.
Thanks to Dr. Thinker for letting me know about this episode!