Resistor for maximum water electrolysis
You need CURRENT source for electrolysis.

If you'll use 120v then several volts will be use to break H2o molecule and other hundred+ volts will just heat water or resistor.

You need low voltage source (don't know exactly how much for water, several volts) that can give lot of ampers. And also electrodes with large area

Google for electrolysis equation, mass of electrolyzed material depends on current, there is no voltage at that equation at all