Mindscrape, As my previous post says , the plates do not have to be in contact with the water. You can coat the plates in a non-reactive, non-conductive coating then apply the use of water as a dielectric. The conductivity of the water would not be a factor. Also it doesn't have to be pure...
Why do so many think the plates have to be in physical contact with the water? Capacitors are used as fuel sensors in the A4 Skyhawk aircraft and the dielectric is the fuel. However in this case the plates are insulated from the fuel. The same would logically be the case in a capacitor using...