Liz Kessler 
Emily Windsnap and the Castle in the Mist 
Book 3

The magic ring that Emily Windsnap – half mermaid, half ordinary girl – finds buried in the sand belongs to Neptune, and he wants it back. But the ring, once on, won’t come off, and an angry Neptune sends Emily’s boat spinning away across the sea. When it comes to rest, she and her best friend, Shona, can see a mysterious castle shimmering in the mist on the horizon… Another magical adventure full of fun and friendship!