Water, especially for larger cities, will definitely be an issue in our lifetime. Aquifiers are being drawn down faster than they can recharge. Surface water is subject to a host of issues such as rainfall, contamination, recreation, conservation and agriculture to name a few.
As cities grow they have to find more and more sources for potable water and unfortunately they are NOT going to get any cheaper to clean. Eventually it will be more cost effective to start using gray water for more non-potable water uses.
Desalinization plants are a nice idea but are a huge energy hog and don't produce a large amount of water. The environmentalists typically fight any new plant anyhow so getting one built can be a chore.
"Any society that would give up a little liberty to gain a little security will deserve neither and lose both."