There's no doubt that with electricity you can split water into Hydrogen and Oxygen, but with as little power as your car can provide, I honestly don't believe that you could make this work.
What does work, and is very interesting to me, is water injection. Using wide-band O2 meters, and tuning software, they run their cars ultra lean, and inject water to curb detonation.
With other modifications to exhaust and combustion chamber, these cars (mostly like 60 hp suzukis and such lol) are averaging over 100 mpg...