I'd completely forgotten about this film, but happened to switch over to BBC1 a couple of nights ago as it was just about to be shown. It actually makes sense now I've seen it from the beginning! It's not really a horror film as such, I'd describe it as more like a dark fairytale - an interesting premise certainly, I hadn't heard this myth before. Is it an actual myth, or just made up for the film, I wonder? Anyway I enjoyed it, the little girl who played Ebrill (the girl thrown into the sea 50 years earlier by her religious fanatic father) was suitably creepy. Hints of 'Ring' as Wirbel commented. The twist was actually quite unexpected and rather great, kind of justifies the title of the film.
The actress who played the mother seemed very familiar to me, I'm sure I've seen her in something else but I can't think what ....
Interesting that this was made by the director of 'Ginger Snaps' - what a brilliant film that is, one of my favourite horror films ever. So witty.

Ice, you were the one most tender with the rivers.
You, the roof of the waves, layer after layer after layer ...