About two best friends, Haley (JoJo) who is moving to Australia because of her mother's work, and Claire (Emma Roberts), who is afraid of the water because her parents drowned on a boat. Claire lives with her grandparents. They find a mermaid, Aquamarine, after a storm that came after a wish that Claire and Haley made at a sleepover they had. Aqua explains to them that if they help a mermaid, you get a wish as long as the wish doesn't violate the laws of nature. They agree thinking that they'll use it to make Haley stay in Bay Bridge.
Aqua wants to fall in love. Her father thinks love doesn't exist and down in the ocean, the mermen pick who their daughters marry. Aqua has three days to prove that love does exist or she'll be forced to marry a blowfish. Aqua picks to fall in love with Bay Bridge's beach club's hottie, Raymond, but Cecelia will do anything to stop her. Haley and Claire help her learn how to make Raymond fall in love her, but in the end, it won't work. He wants more time. Cecelia comes and pushes her off the pier and she falls into a whirlpool that her dad created to bring her back. Her tail pops out because of the water and Raymond sees it.
Haley and Claire go in to save Aqua and Aqua explains to them that they don't get their wish because Raymond doesn't love her yet. Haley didn't want to use the wish on her staying because she realised how much her mom has worked for that job. They tell Aqua that they love her and the storm created by Aqua's father, goes away. They make a wish to keep Aqua with them, but she says she can't but will try to visit. Claire is happy because she faced her fear of the water for Aqua, but Haley still has to go to Australia. They remember that they still have their wish.