Is Ningyo hentai?
English title: Mermaid
No
Rated G (All Ages) on MyAnimeList and not tagged as Hentai.

Movie · 1964 · 1 episodes · G - All Ages
Fantasy
In a fictional place where using the imagination is banned, a boy saves a fish, which surprises everyone by turning into a mermaid. The boy is arrested for imagining this "nonsense", and is robbed of his imagination as punishment. (Source: ANN)
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