Here is what I recall of the film:
It took place somewhere in the woods and there was a laboratory in the woods. I think the main antagonist was a woman but almost alien or a monster of sorts. Like there was some experiment being done on her or by herself.
There was a fight in a rowboat on a lake when a guy attacked a man and woman in the boat and they killed him with an oar and then they saw his back was horribly scarred maybe by radiation or something.
The film is black and white for sure and I saw it on TV in the late 60’s and no later than early 70’s.
Here is what it is not although I thought it might be, any She-Devil or any of the main She- films. I have seen those recently and they are not it.