Is there a spell that will enchant a necklace to turn you into a mermaid when you put it on and change you back into a human when you take it off?

I'm sorry, though I am partially Wiccian and you can say, turn into a mermaid, with LOTS of practice and smarts on the Wiccian way, as far as I know of, there is no way to enchant a necklace in that way. You would need to ask a professional to enchant said necklace for you. However, you could enchant said necklace so you could be a better swimmer. Clear your mind, and hold your necklace in your palms while sitting cross legged. Imagine yourself swimming in the Olympics, swimming gracefully and soaring ahaid of your compedators. Now, imagine that feeling turning into a mist, and that mist being absorbed by your necklace. Wear that necklace whenever you swim.