Well, mermaid spells shouldn't be a surprise after seeing so many people after vampire spells. I guess the appeal of the mythological world can be pretty strong. I would probably choose being a mermaid over a vampire personally.
As I'm sure you can guess, I don't actually have spells to become a mermaid in any literal sense. Whether or not they even exist is another matter, but there is no way to change yourself into becoming one. What I can offer, are a few rituals to help you connect with your water element and embrace the energy of a mermaid.
Diving Deeply Tea Ritual
Bring the energy of water into yourself with this meditative tea ritual. The herbs in this tea are all safe to drink and the flavor should be nice too. A little honey wouldn't hurt the spell if you want.
Use a kettle to heat up some water, and make a cup of tea with the herbs. A strainer or tea ball can be used so you can get the herby bits out. While the herbs are steeping, add the stone to your teapot so it can add its energy to the tea.
After about 15 minutes, remove the herbs and the stone, and pour a cup of tea. Add 2 or 3 drops of vanilla extract. As you breathe in the scent of the hot tea, repeat the following:
Through the water like a fish,
Like a mermaid, that's my wish.
Through the deeps and ocean blue,
That is what this tea will do.
Repeat the words while you envision the underwater grace of a mermaid, and what it must feel like to swim through the wild ocean. Drink your tea while keeping your thoughts focused on these ideas. You don't have to glug it down. Sip and enjoy.
It's not exactly a true mermaid spell, but it will draw in the energy of water into your body.