What was the name of this movie?

Hi, I saw a film a while ago about a few teenagers and a merciless current. Unfortunately I've forgotten most of it 😅 I'm not 100% sure if it was a video or a film. The film was about a group of children/teenagers. And there was a lake with a river running into it that had a scary current. There was something in the lake but I don't remember that exactly. At the end of the film someone found out that the children had been snooping around and hooked one of them up with a belt on a kind of rope and let him hang over the lake to take something out or something. And the cover image looks like a wave on a triangular sign. Unfortunately that's all I remember. Anyone have any ideas?