It's a love story between what I think is a fairy and a non fair. I know that it's not the "Sea Prince and the Fire Child."
The female love interest was a fairy. There was a crack in the bottom of the ocean that was forbidden to be crossed because it would unleash something evil. The main boy character crossed the crack. The fairies were leaving because of the evil that was unleashed. They left on these weird plants that uprooted from the sea bed and had tops like helicopters that were going upward. I think the girl left on one, and the boy was left behind.

I have been searching for this movie for years... if you have any idea, PLEASE PLEASE PLEASE let me know. It's driving me nuts and I feel borderline obsessed with it. Thanks!