Why was the result of drinking river waterr?
Chapter 13 three men in a boat

Falling  short of drinking water,they go up to the lock-keeper to ask for some. the lock-keeper recommends them to fetch some from the river. They finally get some from a cottage nearby. He recalls an incident when the got some water from the river, boiled it, prepared tea and start drinking it. It was during the course of drinking tea that they saw a dog swimming in the river. the three felt disgusted and discountinued drinking the tea