Yes, yes, yes, I wanted to be a mermaid. I first wanted to be one before I even knew that girls didn't have willies, so that can't have been the reason at the time. I was only given boys' comics, but one of them was the first Giant Superman Annual. It was dated 1960 but I was in England and US comics usually reached us several months after publication. There was a story about Lori the mermaid. I particularly remember that issue because there was a lot in it I liked, including a Supergirl story.
The next "I want to be a mermaid" phase was after seeing Miranda, starring Glynis Johns. By then I knew that I really was a boy but I'm still not sure that I was consciously or subconsciously choosing a fishtail over legs and a willy.
As an adult, I thoroughly enjoyed Splash and Disney's Little Mermaid. I actually have a number of plastic toy mermaids, including an Ariel, that I bought as an adult. Childish, I know. I should have at least gotten a porcelain one.
It is said to be a trans girl thing. I once read a book called The Erotic World of Faery, in which the author claimed that Hans Christian Andersen was a trans woman, and that the mermaid's wish to lose her tail was an expression of Andersen's desire for emasculation. Youtube's Maya was once called Transmermaid and I think that Jazz Jenning's family have a business selling mermaid tails. There is also an organisation called Mermaids UK that provides support for transgender children.