Mako: Island of Secrets (Secret of Mako Island) (2013)

Zac is a teenage boy who decides to camp on Mako Island, unaware that three mermaids, Sirena, Nixie and Lyla, who are guardians of the island, are watching him. That night, when the full moon rises, the boy comes into contact with the magic water of the Moon Pool. The following morning, Zac wakes up discovering he has become a merman with a fish-like tail and the ability to control and manipulate water. His new-found merman abilities will cause problems for the mermaids.
The three mermaids, curious about living on land, are transformed into humans while in the moon pool during a full moon, now having the ability to live like humans.