Witches would be the obvious choice. Tradition dictates that the sinking test will root out evil, whilst providing a watery spectacle for all the family. Carpenters are kept in gainful employment knocking out ducking chairs, and stall holders can set up shop along the banks of the river, providing sustenance for the gathering throng.
Alternatively, fluoride would seem like a sensible addition to the public water supply. Although, there are people who argue against this nanny state policy. People can buy their own fluoride in a tube and floss while they are about it, they might say.
I am in favour of substances being added to our drinking water, if it is for the greater good. Witches are not included in my own list of substances that do well in water, but utility companies adopting a utilitarian stance? Why not?
In fact, I think that during the gloomier parts of winter, a daily low dosage of Prozac alongside the fluoride wouldn’t hurt. What about some vitamin C? Announcements could be made directly to customers by their medium of choice, or an online message board could inform the squeamish about which tipple has been added to the H2O that morning, so that they can rush to their nearest store to stock up on flagons of expensive designer liquids.