Mermaids aren't real. There are no spells to turn you into anything - you can't become a mermaid, or dragon, or vampire, or any other mythological being. See, none of those exist. As for spells to make them exist, well, if there were spells that worked at making you a mermaid, don't you think those same people would've figured out a spell to win the lottery, or taken over the world, or something else along those lines?
I mean, heck, why be a mermaid when you can be Emperor of the entire planet? Let's get some logic processes going here, shall we?
Seriously, I am getting rather annoyed at all the wannabe werewolves/vampires/ now MERMAIDS of all things....how come no one wants to be a manticore, or chimaera, or hydra, or kraken, or a minotaur....I mean, come on, minotaurs are pretty badass, I guess people have something against having a cow's head....or maybe there just haven't been enough books or movies made about them lately.....
Report away, dear one. The fact remains that if it has special powers and you've seen it in a movie, it doesn't exist. Mermaids included.