The Little Mermaid (2018)

This 2018 The Little Mermaid isn't an adaptation of the Hans Christian Andersen story, but it does use it as a basis. The film's animated prologue recounts the Andersen fairytale, but instead of a sea witch asking for the little mermaid's voice as upfront payment, a wizard asks for the little mermaid's soul if she loses. When the little mermaid's beloved prince marries another, the wizard wins and enslaves the little mermaid.

From there it moves to the live-action portion of the film. In the 1940s (I think, feel free to correct me in the comments) an investigative journalist named Cam travels down to Mississipi to check out a carnival that's been said to have a "mermaid" as well as "magical healing waters". Cam brings with him his niece Elle, who's been ill since she was a child.

At the carnival, ringmaster Locke unveils the mermaid (Poppy Drayton) in a tank. Cam is a skeptic but Elle is a believer, so only she is mesmerized by the mermaid. Cam feels the whole thing is a hoax, but the more he looks into it the more finds signs that something sinister is going on. Along the way, Cam gets to know the mermaid, Elizabeth, as a person and becomes just as determined as Elle to help her.

It's a small story about believing in the impossible, though surprisingly apart from the prologue it doesn't use the Andersen elements much despite technically being a "sequel" to that story, i.e. Locke owns Elizabeth's soul, but that's pretty much the only connection to the fairytale. Instead the film uses its own lore of mermaids, among them that Elle turns out to have a connection with mermaids herself. On the whole it's a self-contained story of how a captive mermaid breaks free thanks to the love and courage of two humans who believe in her.