But the fishermen on board the boat mistake Cleo for a shark and catch her in their net.
Emma and Rikki rescue her and Cleo is no longer afraid to swim in the water.
When he realises what he has done, Zane works with Lewis to free the girls who trick the scientists into believing they have given up their powers during a lunar eclipse.
After further experimentation, they discover they also have unique supernatural powers over water.Emma can freeze water, Cleo can control the shape and volume of water, and Rikki can boil water.Realising that the pool, which has an underwater tunnel connected to the ocean, is their only way out, the girls step into the water and find themselves bathed in the light of the full moon as it passes over the top of the volcano.Little do they know, as they swim out of the tunnel and are rescued by the water police, that they have undergone a magical transformation, and their lives will never be the same again.The girls enlist Cleo's long-time friend Lewis Mc Cartney (Angus Mc Laren) to try to determine why this has happened to them and help them keep their new identity a secret.
The series focuses on the girls dealing with everyday teen problems while trying to cope with their newfound abilities.
A worried Emma cancels her annual sleepover party when she realises the risk of growing a tail in front of all her guests.
She and Lewis travel back to Mako Island to try to find out how the girls were transformed into mermaids, and Emma finds a mysterious locket at the bottom of the Moon Pool, and thinks that it may be important.
Ten seconds after they touch water, they turn into mermaids.
Cleo and Emma discover that they have powers to control water.
Emma and Rikki use their new-found abilities to free hatchling sea turtles caught in fishing nets.