My imprinting version was Splash (1984), in which the change is painless and flexible.
My defining childhood mermaid media was Liz Kessler's The Tail of Emily Windsnap, which also had a painless + flexible mermaid transition, although there the protagonist was, in the immortal words of Lady Gaga, just born that way, so I didn't feel like that counted for this prompt. (Although she didn't discover her mermaid powers until middle school, because what kind of MG adventure would it be otherwise?)
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My defining childhood mermaid media was Liz Kessler's The Tail of Emily Windsnap, which also had a painless + flexible mermaid transition, although there the protagonist was, in the immortal words of Lady Gaga, just born that way, so I didn't feel like that counted for this prompt. (Although she didn't discover her mermaid powers until middle school, because what kind of MG adventure would it be otherwise?)