Seriously: water, power, and other critical utility infrastructure providers are not a low density/low volume market. There are large enough economies of scale such that there should really be no discussion here. There should be a separate physical network for these industries.
Air gap the network, heck, develop and mandate totally new hardware interconnects to ensure some moronic PHM or more likely brain dead network admin isn't physically capable of connecting COTS hardware to SCADA hardware.
There is absolutely no reason for any of this stuff to be directly accessible to the public internet, the utility provider can very well have some data diode http://en.wikipedia.org/wiki/Unidirectional_network/ to provide metering information on the public internet side, but there absolutely should be no bidirectional links between the command and control network and the public internet
There would be no astronomically expensive software validation necessary if these industries were mandated to require Hardware level compartmentalization, which funnily enough a custom hardware solution would be orders of magnitude cheaper and deployable now rather than some pie in the sky (never going to happen) software based solution that the "Tube" worshiping ludites in Washington think can actually be created