Fear of drinking water is a classic paranoia, especially since in medieval times or earlier it could be quite difficult to come by a clean water supply (just imagine what it was like in the days before water filtration and the only river within miles had thousands of people throwing all their waste, including biological wastes, into it upstream) and thus a well could be the center of life for miles around. If anything happened to it or anyone tried to contaminate it, you'd never know until it was too late...