Read Deuteronomy 18 to see what it says about divination.
But is “water-witching” what the Bible writer meant by divination? I believe that refers to attempts to predict the future by magical powers, usually involving contact with spirit beings.
Subconcious detection of minute magnetic anomalies, or however you prefer to think this process works, does not necessarily involve anything of the kind.
That said, I agree with those who point out that, despite all the anecdotal evidence, there has never been any proof of such abilities found using controlled scientific tests.
Possibly the ability hides when somebody pulls out a notebook. :)