The Rainbow Pool Mermaid Curse Series, Book 3.

Taran, the evil mermaid Queen is determined to get her hands on the silver pearl that will make her more powerful than ever. Only Lizzy, Kes and their friends from the undersea world can stop her. But when Taran takes a very special hostage, Lizzy and Kes are faced with a terrible choice. Must they give up the precious pearl!? Or do they have the courage to challenge Taran and defeat her?