Linda Chapman 
Not Quite a Mermaid: Mermaid Island [EPUB ebook] 

Elecktra is a human mermaid — a mermaid with legs instead of a tail. Abandoned as a tiny baby, she was found by the merpeople of Mermaid Island. They decided to adopt her and gave her sea-powder so that she could live and breathe underwater. Now aged nine, Elecktra is always getting into trouble, usually because of her desire to have adventures. Whereas most of the merpeople are content to stay within the safety of the coral reef that encircles Mermaid Island, Elecktra longs for excitement. She often leads the other merchildren astray but then saves the day through her resourcefulness and courage.