Don't have time to write a full review. I probably will later.
In short though, this book is really slow and dull. The characters lacked any luster for me and the most unique ones were introduced at the end. The main character, Hannah, was boring, too innocent, and just not easy to relate to. Plot wise, it was awful. I knew the ending before I even read the book, simply based on the description. There's no guessing involved at all. The only interesting part is the younger girl in the house that Hannah works at, with her creepy cat. But, even that doesn't make up for the fact that the book is incredibly slow, and the whole thing leads up to an event that the reader should know is going to happen no matter what. There were a few characters and ideas that the author could have expanded on to make the book actually interesting, but she didn't even do that much. Honestly, I can usually find something in a book that I liked. There wasn't much in this one. I finished it with a gut feeling that I had wasted my time and money on the novel. My advice is to not pick this one up. I have no clue why it's a Young Adult book. If anything, I would put it in the children's section (as younger kids are the only ones that might find this interesting). If you want a mermaid story, go read Siren by Tricia Rayburn or Mermaid Park (this isn't exactly ABOUT mermaids, but it seemed to satisfy me when I was younger and extremely interested in them) by Beth Mayall. Apparently there's a sequel to Hannah out...? Hopefully it's better than this. I personally don't intend on finding out.