I read that water molecule steal electron from oxygen molecule but the next sentences onward is talking about the benefit of negative ions for health, what is the cause of negative ion production in waterfall and does it also apply for rain?
Whenever water bodies strike the surface very hard they release ions due to their collisions. These ions according to some source don't really affect you and act as placebo.
Source for this:https://youtu.be/ZQ--scjcAZ4
You should definitely see this wonderful video I think it'll answer all your questions as it shows an experiment too.
I think that helps:), Thanks