That ist no quite correct.
It's just, that every individual has his/her own sensitivity to these fields. So while some people/bodies react very positively, others react negatively and some even don't react at all.
I myself am able find underground water streams, with plain copper rods (simple bent wire will do the trick)
My late father wasn't at all, while my little sister could to a lesser degree, and a good friend of mine was far more sensitive, than I was and the rods would react far more intense.
I never made a big fuss out if it, or tried to sell my services. I only discovered it whe we were looking for water (for a borehole), while living in Afrika with someone present who had exactly this ability.
There is nothing magical about it. It's simply your body being responsive to a magnetostatic field being created by the flow of the water. Because it doesn't work with "standing" water....
So just because you haven't seen it happen before, doesn't mean it doesn't exist.
No magic, just plain natural science