Continuity mistake: Cecilia and Raymond are on the end of the dock and Raymond turns around and runs. Cecilia then says her line, turns around and runs. In the next shot the two girls are running towards the end of the dock. You can see Raymond running towards them but Cecilia is gone. She should be in the scene running the same way. (01:28:05)
Continuity mistake: When Cecilia is at the water tower looking for Aquamarine, one or both of her earrings are missing. when she gets back to the beach party she now has both of them on.
Continuity mistake: When Aquamarine is in Leonard's house after he rescued her from the water tower, the tuckshop lady comes down the stairs outside with a basket of washing and walks away. The camera then goes on Leonard and Aquamarine again, before showing the tuckshop lady coming down the stairs again with the same basket of washing and walking away.
Continuity mistake: Near the end, when Aquamarine (as a mermaid) is talking to Raymond on the surfboard, her wet hair drastically changes between shots.
Continuity mistake: During the balloon toss, when Raymond gets wet, the water spot on his shirt changes location three times.
Continuity mistake: When Claire and Hailey are watching Cecilia and her friends talk to Raymond, a man and a woman in a white bikini pass by in the background, but after a quick cut to Claire and Hailey, the couple walk by again.
Continuity mistake: When Aquamarine says "Boo!" to Claire and Hailey in the swimming pool, you can see her fin, but in the next scene when Claire and Hailey scream, the fin has gone back under the water.
Answer: Most likely, Aquamarine and her father had argued multiple times, about her not wanting marry the boy that he chose for her. She was going against tradition. Claire's grandmother probably saw storm signs caused by an earlier argument. Then Aquamarine's father got angry enough to make the big storm.