Was it The Giant Eel? It would have been dubbed on the Film Festival.

https://www.dailymotion.com/video/x15kmxg

I remember something like this as well, from the seventies, they played it over & over many a Saturday afternoon. I remember it seemed to be a loch or lake in Scotland? Ireland? They had some kind of hideout, like a tent off of the road surrounding the body of water. I remember a girl & a boy.