The mermaid looks at these obstacles and doesn't give a single damn. She drinks the potion, enters the world of humans, and ... everything goes to shit, because otherwise we wouldn't have a compelling story. The prince falls for her for a while, but he wanders off to marry a random princess, having ironically mistaken her for the woman who rescued him from the sea.
Right as it seems that the game is over, the mermaid's sisters enter the fray. They too have visited the sea witch, and negotiated a deal in which they sold their hair for a magic knife that can return the mermaid back to, uh, a mermaid. All she has to do is kill the prince who broke her heart, and she can go back to living her life.
So be thankful all your ex does is make passive-aggressive Facebook posts.
She can't do it. Instead she accepts her loss gracefully, and decides to go in peace. She jumps in the ocean and dissolves into foam ... only to immediately resurrect as an air spirit. Other air spirits inform her that she's too awesome to die, so she now gets a chance to gain a soul and an afterlife the hard way: performing 300 years' worth of selfless acts and good deeds as an unseen "daughter of the air."
It's weird. A little convoluted. Suuuper Danish. But it's a solid story, and the OG Mermaid always decides her own fate, never once losing her own values or sight of her target. Plus, she's aided by a number of other strong female characters along the way. And all that's before she joins a band of ethereal ladies zipping about Earth's atmosphere and helping people in secret.
Walt Disney Studios
Which would kind of render "Under The Sea" moot.
Maybe it's time for a reboot. She'd fit right in with the other superhero flicks.