The solution as Torie already noted is to price water in the market and then subsidise it for those who can't afford it. As water grows more scarce declaring it a human right will not change the usage that was drawing down the supply in the first place. Pricing it so the heavy users reduce their consumption will.
I'm not sure how you do this for places where pipes are already in the ground. Even the private water utilities that exist around here are going to undercut water prices purchased in bottle form. Unlike electricity or gas there isn't a deregulated supply market that can supply water to different distribution systems.
They heaviest users in my area are food manufacturers, and their water prices are passed along to the consumer with little impact on consumption behavior. Residential users are typically faced with sprinkling bans that when enforced are effective at dropping the large increases seen on hot summer days. There are large price differentials between communities and only a few industries make their location decisions based on the price of water.
