There are several Exchange Traded Funds which have water portfolios. One is PowerShares Global Water (NYSE:PIO), another is PowerShares Water Resources (NYSE:PHO), FirstTrust Water (FIW), and FirstTrust Global Water (CGW).
There are also several mutual funds which restrict their portfolios to water. There is Kinetics Water Infrastructure No Load fund (KWINX); Sustainable Asset Management’s Sustainable Water fund (SMWNX); First Trust ISE Water (NYSE:FIW); PFW Water fund (PFWAX); Allianz RCM Global Water fund (AWTAX); and Calvert Global Water fund (CFWAX).
There are several individual water stocks, but I’d recommend that you purchase a portfolio of them. If you wish to pursue individual names, you can get many of them if you view the ETF or mutual fund’s portfolio.