Waiting Here - Kate Alexa

Series three sees the departure of Emma, who has left to travel the world with her parents, and the arrival of Bella Hartley, who has been a mermaid since the age of nine. Rikki and Cleo become friends with Bella, but are soon beset by a mysterious tentacle of water with a connection to Mako Island. A new boy, Will Benjamin, also arrives and become friends with the trio when he discovers that they are mermaids. The girls learn that Earth is in the path of a comet that could destroy the planet. Using their powers, they manage to stop the comet from colliding with Earth.