FTLinmedium, you cant have a string at that part of the river, period.
There is no part of a river that a string with a fish on it can not get to, period. Water currents do strange and counter-intuitive things.
If you have faith that it was not a fishing light, that's your right and I can't argue with that. But everything you described is consistent with such a light.
If you would like, you could take a picture of the location (in day time) and post it here so we can see more clearly what you're talking about.
I saw this light thing and the way it behaved was not in a way something on a string would have behaved. This one thing I am for certain.
You shouldn't be too certain about that, but it's your right if you want to be. A string in the water does weird things. One with a fish pulling it, ever more so. Everything you described is consistent with it being on a string.
Your feelings have a great influence over what you see. I beleive you could have seen the reflection of a planes light on the water and your feelings and belief in the supernatural was all that was needed to make it seem like you saw something amazing.
I can't tell you how many times, being near sighted, I've seen somebody walking, only to get closer and realize it was a sign post. Could have sworn every time it was moving, arms swaying, etc. Our brains fill in an amazing amount of false information- it's why optical illusions work, and it's the mechanism behind hallucination.
Generally speaking, I don't trust my eyes- and nobody, no matter how good his or her vision is, should trust human eyesight. It's based more on expectation than what's actually there half the time.