A Ring of Endless Light

TV movie A Ring of Endless Light isn't a mermaid film, but there is mermaid subtext in it. The film proper is a coming-of-age story about a girl named Vicky Austin (Mischa Barton), whom with her siblings are visiting their grandfather for the summer.

Vicky is a writer and a dreamer, and over the course of the film learns that she has an unexplained ability to telepathically communicate with dolphins. That connection is symbolised by two dream sequences where she dreams that she's a mermaid, and in the final climax of the film she herself is "seen" as a mermaid by her friend when she rescues him from drowning. These mermaid sequences are short and dream-like, but it's impressive effort by Mischa, who was fifteen or sixteen at the time, to swim and act in these sequences herself.