Somebody Out There - Kate Alexa

Rikki Chadwick, Emma Gilbert, and Cleo Sertori are three sixteen-year-old Australian girls who find themselves stranded on the mysterious Mako Island. They eventually find themselves in a pool under a dormant volcano just as a full moon passes overhead, bathing the pool in light. The girls are rescued and brought back to shore where they return to their normal lives, but soon discover their lives have become anything but normal. The girls find out that seconds after contact with water, they transform into mermaids. After further experimentation, the girls also discover they have supernatural powers over water. The trio enlist the help of Cleo's friend Lewis McCartney to help them keep their secret.