There are spells like that-but they will never work.
Mermaids don't exist in this plane of existance, and if they do they were probably caused by drugs in the 70's that caused webbed feet. Also, even if there were mermaids in this plane of existance I highly doubt that they would be transformed everytime they touch water- it's just not realistic. Please try to use the right side of your brain when thinking about magick- yes it does exist, don't get me wrong. But, it will probably never turn you into an elf, a faerie, a mermaid, a ogre, or shrek while on earth. Magick is logical, and a science in itself, but it cannot make anything happen, it does not defy the laws of the universe.