When Aquamarine talks to Raymond for the first time, the blue streaks in her hair are bright and prominent, but when she goes to the hammock complaining that Raymond doesn't love her, the streaks have disappeared
When Hailey and Claire are hiding behind the umbrella in the beginning as they watch Ray, Hailey is shown laying down as the camera faces her, but when the camera is behind her she is shown propped up on one arm.
After Cecilia pushes Aquamarine back into the ocean on the boardwalk, she turns to walk off the boardwalk. But when she would have to pass Claire and Hailey to get off the boardwalk, she doesn't. We only see Claire, Hailey and Raymond, and Cecilia is nowhere in sight.
The movie is set in Florida but really was filmed in Australia. One scene in particular shows the girls reading magazines. One magazine is TotalGirl which is only sold in Australia and the areas around it.
Raymond puts an adhesive bandage on Aquamarine's scraped knee without first checking for debris, or offering to flush/disinfect the wound. A trained lifeguard would know to do both of those things, as part of basic first aid training.
Aquamarine is rescued from the water tower during the night by 'a man with a ladder'. But how would she climb down any ladder whilst she still had her mermaid tail? And she would be too heavy for anyone to carry down.
Aquamarine claims that one drop of water will turn her legs back into a tail, so she has to avoid it during the day and in public, in order to keep her secret. But throughout the film she interacts with water (spitting it over her lap from a cup, riding in/clambering across pedalos on a lake, walking along the beach/pier, attending an event where water balloons are being thrown) without any apparent fear of or wish to avoid it.
When Raymond swims out to save Aquamarine from the storm, Aquamarine gets up on his surf board. They start to talk and one shot of Raymond shows he has several drops of water on his face. In the next shot his face is bone dry. Between those two shots it doesn't show him wipe his face dry.
When Aquamarine is talking with Raymond on his rescue board near the end, she is seen with her hair covering her chest. On the shots showing Aqua close up, she appears to be nude, but in the shots showing both of them, you can see a neutral-colored bathing suit between her hair.