I would think any source of water that didn't contain guinea worms might have been revered as heaven-sent.
I've dranken river water often, probably at least as contaminated as the ancient nile, and not become ill.
(Not always voluntary, by the way, sometimes you can't help getting a gulp when your canoe flips over. )
Considering that they wouldn't have known what made them sick *, and it occurred only on occasion, they didn't draw the conclusion.
*(think of the cholera epidemic, where people continued drinking from the same water pump, and becoming sick, yet not coming to the conclusion that the water was contaminated)