Mako: Island of Secrets (2013)

Sirena, Nixie, and Lyla, young mermaids of a pod residing in the enchanting waters of Mako Island, are tasked with safeguarding the mystical Moon Pool. However, during a full moon celebration, their playful antics lead to a disastrous oversight. When Zac, a curious sixteen-year-old human, accidentally enters the Moon Pool, he forms a unique bond with the island, gaining a fish-like tail and extraordinary abilities. As a result, the mermaids are expelled from their pod, and to regain their status, they must venture onto land, retrieve Zac's powers, and reclaim their place among their fellow mermaids.