The mermaids appeared in the premiere of Season 3, "Heart of the Truest Believer". They were depicted as dangerous in Neverland, though affiliated with Peter, like in the original novel. Like the mermaids of Pirates of the Caribbean: On Stranger Tides, these tend to take an alien, predacious, and perhaps even villainous role, though their individual personalities and moralities may differ. Like Aquarianne from Disney's Adventures of the Gummi Bears, some carry a conch shell which can be used as a magic flute. They are perhaps the most powerful ones ever produced by Disney; gifted with magic so great it can control the very weather itself.
While Snow White and her companions are sailing the seas of Neverland in Hook's boat, their ship is attacked by mermaids (who strike with such zeal that they are first mistaken for sharks, sea monsters, and even a Kraken). Once they have clouted the ship's hull with their tails, Regina prepares a fireball spell while the rest of the crew tries to drive them away with artillery or capture them with a fishing net. All of these tactics succeed; they are driven away from the ship, save for those who might have been slain by the fire and the one who is captured in the net.
This mermaid prisoner is fiercely defiant of her captors, asking/demanding that they let her go and stating that it is not she who dooms them, but they themselves. While the major characters argue over what is to be done with her, she proves her point by summoning a tempest. As their ship is tossed about upon mighty roiling waves, her crew becomes rain-soaked and desperate. Regina then casts a spell which turns her to wood yet fails to stop the storm, leaving her and her companions to brave the wind, lightning, and driving rain summoned by the malevolent mermaid.