Michael Dahl 
Marshmallow Mermaid [PDF ebook] 

At Halo Nightly’s school, the swimmers are disappearing from the boys’ swimming team. No one can figure it out, until Halo starts investigating. A deadly mermaid lives in a cave connected to the school swimming pool. Halo will have to use her special sweets to turn into various underwater fauna, such as a hammerhead shark or an octopus, to defeat the hungry girl-fish!