H2O: Just Add Water Season 3

Zane has a surprise: his dad bought the café and made him manager. He renames it after Rikki, whom he makes a full partner. Tone-deaf Nate nearly ruins the first life music performance, but Lewis saves with Isabel "Bella", whom the other mermaids accept after discovering she's one of them as they all abused their power to hassle playfully taunting surfers. Hunky local nature lover Will goes diving around Mako island. He discovers the magical cave, but it's a full moon and the serpentine water column knocks him out before escaping to attack the others. It grabs Rikki, but the other mermaids follow and liberate them. Will is totally confused, but unharmed.