G'day. Enthusiast open data advocate in Australia here. IMHO, security issues are worth exploring, but they usually end up as "Yes, a terrorist could do something nasty to the water supply. But having a detailed map of the network and hydrant locations doesn't make that significantly more likely". If anything, it could in some cases do the reverse, by making it easier for a wide range of people to access the information and possible take some kind of preventative measures.

An example of one of our councils' water assets: http://nationalmap.gov.au/#share=lY9zXc