i'm interested to know why people always 'need' to be a mythological creature, i'm certain you just 'want' one, but thats beyond the point.
unfortunately for you, magick is a force of nature used similar t prayer to help us through lifes rough patches. it cannot contract nature, since it is a part of nature, and therefore physical transformations will never work. accept the fact you are human, then perhaps decorate your home with mermaids, research them, incorporate them into your life somehow, but realize you cannot become one especially by magick.
You can't become a mermaid, it's not possible, but even if it were, why would you want to? Every human is special in his/her way, we all have our talents. Transformation isn't one of them though. We are all bound to the Laws of the Physical Dimension. No matter how much we try or how much some of us protest this, it's not going to make a difference.