I think this is someone trying to make a quick dollar.
First of all, the engine of the car is used to recharge the cars battery. If you use energy from the battery to electrolyse the water, the engine will have to work harder to recharge the battery.
To continue, it is very difficult to simply electrolyze pure water. It is not, in and of it self, a very good conductor. If you use an electrolyte such as NaCl (Salt) to enhance the conductivity of the water you will produce Hydrogen and Chlorine gas. Not the Hydrogen and Oxygen that is needed.
In my opinion, if you could do it safely, yes adding hydrogen and oxygen to the engine would increase the amount of energy released when the spark plug ignited. However, the amount of energy required to create this hydrogen/oxygen gas would completely negate any benefits from the extra power and would end up costing you more efficiency. Sorry but this will not work.