According to my guide in Edinburgh, accused witches were also dunked in the Norloch for the crime of having red hair. Which is a wee bit macabre-funny, given that Scotland and the ginger-haired look (think bearded clansman on tartan tin) are almost synonymous in my mind.
Sometimes the accused did drown after it became clear she was innocent, yes. I don't think there was a prescribed length of time to wait, which made it dependent on the kindness(-less) of one's neighbors (in many cases the same people who were accusing you).
I wonder, during witch-bobbing trials, at what point of the victim's sinking, it finally dawned on the mob that she was, in fact, innocent. Did they have to wait until she had actually drowned, to declare posthumous innocence?