Shitting in the river will make you sick, but it's also the low-class thing to do, it's unfashionable, it's unattractive to the opposite sex, it makes you look foolish. Toilets are respectable, they're modern, they're stylish, they're a status symbol. You don't have to invoke superstition to create a taboo.
If you want people to boil their water, just provide subsidised tea and let the addictive properties of caffeine do the work for you.
So, they made iron lumps in the shape of fish (a local good-luck symbol) and handed them out - increased compliance a lot.
But in their case water being boiled was something you did when you were sick. This already had them boiling water for sick people, the sick would get better (to some extent confirming their belief), and they'd go back to cold dirty water.
Find out how to explain it in their existing culture. Curses, uncleanness, etc. There's probably SOMETHING responsible for misfortune, death, and disease.
Claim that there is a method which helps battle this, it involves purifying the water with a strong fire for this duration of an hour-glass. Once it is cooled it is, for a short time, safe to use.