I remember watching what I believe was an animated movie as a kid in the 80’s. Seems like it was about a mermaid. Remember the ending was sad and turned to “real life” where it showed she had been turned into stone; underwater I think. I can’t quite recall and not sure if my memory holds up after all these years.
Could it be that you're thinking of the 70s anime version of The Little Mermaid, Hans Christian Andersen's The Little Mermaid?
.... As the sun rises, her body turns to foam and ascends into the sky. The prince realizes that Marina was the girl who had saved his life, and grieves her death. Marina becomes a daughter of the air and her spirit lives on in heaven for her self-sacrifice.
The movie fades back to live-action in Denmark, and the narrator expresses belief that the mermaid princess has become one with the sea and waves and her love and courage forever lives on. The movie ends on a still shot of the Little Mermaid statue in Copenhagen.