The Secret Mermaid: Enchanted Shell by Sue Mongredien
Usborne Publishing ISBN 9780746096154
Far out in the ocean live the Shell-Keeper mermaids. While the rest of the world sleeps, one little girl joins them for some magical mermaid adventures. Molly is just like any other girl when her family moves to Horseshoe Bay. But then her gran gives her a pretty shell necklace and Molly is amazed to discover it is enchanted...and that she is now the secret mermaid.