Welcome to Spells of Magic, dear. I do hope you find all the answers you seek here, and find those that wish to find the same you do. However, I must say that physically becoming a mermaid is impossible. You cannot change your body structure that much, and even if it were possible, your body would go into shock at the changes occurring. If you wish, you can ignore me and keep searching for your mermaid spell. I am simply wishing to save you the later heartache of finally learning the truth. As far as the increase in magical power, I do not have an answer. I wish I did. If I find one, I will send you a mail.
Once again, welcome.