When I boiled water with AC, it was exactly the opposite, as the water got hot, the resistance decreased and more current flowed, until the point that this snowball effect blew my transistor.
I can put 1-2kV DC pulse train in tap water without any problems, but the current peaks are insane, about 10-20 amps, pure electrolysis. If you increase the voltage more, more current will flow. The water gets super hot after a while and makes a lot of cloudy gas.
The more conductive the water is, more current you need to reach high voltage peaks, thats why in distilled water is easy to do. You need a driver and supply that can feed many amps.