[Mr-X]

I'm trying to find a movie I started to watch ages ago on tv, I can't for the sake of me rember it's name and I can only rember the first 5/10 minutes of it. It was in colour but had a old look to it maybe late 70's early 80's. The start was a women going to like a peir at a lake then stripping off to go for a swim, she was swimming in the lake and I'm sure there was something amongst the reeds that grabbed her or tried too. I know that's not a lot of information but that's all I can rember about it does anybody have an idea of what this film could be?

Oh she had brown or black hair and was slim if that helps.