The CDC makes a clear warning about E. coli and water – especially, unchlorinated water: “Don’t swallow water when swimming and when playing in lakes, ponds, streams, swimming pools, and backyard “kiddie” pools.”
E. coli O157:H7 bacteria and other pathogenic E. coli mostly live in the intestines of cattle, but E. coli bacteria have also been found in the intestines of chickens, deer, sheep, and pigs. A 2003 study on the prevalence of E. coli O157:H7 in livestock at 29 county and three large state agricultural fairs in the United States found that E. coli O157:H7 could be isolated from 13.8% of beef cattle, 5.9% of dairy cattle, 3.6% of pigs, 5.2% of sheep, and 2.8% of goats.  Over 7% of pest fly pools also tested positive for E. coli O157:H7. Shiga toxin-producing E. coli does not make the animals that carry it ill. The animals are merely the reservoir for the bacteria.
The Walkerton E. coli outbreak was the result of a contamination of the drinking water supply of Walkerton, Ontario, Canada, with E. coli and Campylobacter jejuni bacteria. However, there have been countless other outbreak linked to lake water. In the Walkerton outbreak, the water supply was contaminated because of improper water treatment following heavy rainfall in late April and early May 2000, that had drawn bacteria from the manure of nearby cattle used to fertilize crops into the shallow aquifer of a nearby well. The first reported case was on May 17, 2000. The contamination caused gastroenteritis and sickened more than 2,000 people and resulted in six deaths.