Who ever came up with that test was a genius. If you are dropped into a pool of water 95-99% of the time you will be "Accepted' by the water and thus be shown you were NOT a witch. If the village priest put on the local Iron Cross on you, it would increase the chances of you NOT popping up and being rejected the water.
People knew people floated, thus by saying people who are NOT witches would go down in the water made "Common Sense". There is a story of the Medieval Bishop who knew the background of dunking and agreed to it for himself when he was accused of being a thief. For a month before had in practiced in his castle and every time in was "accepted" by the water, then on the day of the actual test he popped up like a cork. It is one of the few times when someone actual failed the test (He then admitted to stealing and asked forgiveness).
Now, the dunking test was NOT a test if you had been a witch, but that if you were INNOCENT of being a Witch today. i.e. if you had never been a witch, the water would accept you, if you had stopped being a witch and God had forgiven you for being a Witch, the water would accept you. Other crimes were also tested by Dunking (as for example the above Bishop). Being hit was a hot iron and seeing if you hand festered in three days was another popular test. Most people's hand will fester but on the fourth or later day after being hit with a hot iron. When King Stephen of England was asked to used that test on some peasants who had stolen wood, he was shocked that all of the peasants where, by the test, either innocent or God had forgiven them for their crime. King Stephen then commented he would NOT be as Forgiving as God when it came to such crimes (but accepted the decision of the hot irons).
The Salem Witch Trials refused to use dunking to test for witches, on the grounds witches were known to defeat such tests. Instead we had spectral evidence used to show that the defendants were still attacking the accusers as could be seen by what the accuser said and did in court.
http://news.lib.uchicago.edu/blog/2012/10/29/the-salem-witch-trials-a-legal-bibliography-for-halloween/
Once you understand the dunking test, you see how it lasted as long as it did. The village leaders would bring out the accused and have her or him dunked into a stream. When the water "Accepted" the victim he ordered the people of the village to save the victim for it was clear he or she was innocent. It was a way to shut up accusers by showing "Clear evidence" that what they were saying could not be true. The dunking test was a way to defeat "Common Sense" when it was on a rampage.
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