Not exactly OT (skipable):
I recently wrote a paper for my masters program that addressed this issue to some extent. In my research I learned that SCADA is considered secure because it is only used in indistrial applications, meaning the programming language is unique to SCADA and would require purpose built programs written in SCADA to do anything to those systems. I found examples like the famous Aurora Experiment, as well as an incident in Australia IIRC. For those that don't know, the Aurora Experiment was done at the Idaho National Labs where they sent a command to a diesel generator (the BIG kind, like enough to power a small town)that caused it to literally destroy itself. We are talking about chunks of it flying around and black smoke spewing from the exhaust, not something you can just start up after a quick repair. The other incident was not an experiment, but a disgruntled waterworks employee that had been fired that used a wireless connection to back up the sewers, causing significant damage to the surrounding area. However, this was all a few years old I wasn't able to get much CURRENT information on the security, so hoped that maybe they had improved things! Well Spookadelic and JC8 shot that hope all to hell... thanks guys!
Closer to OT (more skipable):
Anyway, the conclusion that I have come to is that the next big attack on the US is going to be one of two main methods. Either it will be a Mumbai/Oslo, Norway style attack with some IED's and gunmen that lay siege to some poor unsuspecting city; or it will be a cyber attack against the infrastructure.
As close to OT as it's gonna get (entirely skipable after first paragraph):
As far as how prepare for the next attack (there will be one... eventually), I suggest that you not limit it to water since there are numerous systems that are vulnerable. Remember, there are really 4 main parts of our infrastructure that could cause direct physical damage, electric, water, gas and roads. (I didn't list communications since that would be more an inconvenience in many cases and would not lead to physical damage except in extreme cases, and may actually help with traffic!

) The electric grid is actually horribly vulnerable since no power company really wants to own the grid due to the cost of maintenance and updates. It is much cheaper to pay to use someone elses grid. The current grid is outdated and were there to be a well planned attack, it could potentially cause a cascading effect that would knock out power for HUGE areas (PM me and I can send you links to the scholarly articles I found on this). This is not a possibility but a known fact, as can be evidenced to by the recent outages that we have seen that were the result of simple tests and procedures that went wrong. The dangers of the water system was already mentioned: contamination, poisoning, etc. I really don't want to know what could be done to the gas system. After seeing footage of that pipe that failed and erupted into a massive inferno, I imagine it wouldn't be good. Besides, "Live Free or Die Hard" covered that already right?
One scenario that I have posted before that may be just as dangerous in the short term involves the other part of our major infrastructure that many don't realize is susceptible to hacking, the roads. In some big cities, traffic lights are linked by a computer system that allows technicians to alter the lights programs remotely in the event of a malfunction. So what would happen if someone wrote a program that simply made every traffic light green, and then implemented it during the morning rush hour? Accidents across the city, some at high speed, all in the space of a minute. You have instant gridlock as all the major intersections are blocked by accidents. You have emergency personnel tied up across the city, emergency rooms inundated with the injured. The roads would undoubtedly take some time to clear since the tow trucks would all be tied up with cars already, and that is assuming that they can actually get through the traffic to the pileups to move the vehicles. It goes on and on... and all because someone decided to give every commuter their greatest wish, that every traffic light on their route to work was green! (obligatory "careful what you wish for" warning

) But you get the idea. Small hack leads to big problems!
Cease your flatulent winds and listen to my mind numbing expulsions of wicked noise! GRRRR!! Cheese!! - Happy Noodle Boy