Tesla was a smart man!
Comparing electrolosis to cold fusion is a bit of a stretch, I think. Being an electrical engineer, with a few patents, I have a hard time believing this could power a vehicle on its' own. Energy cannot be created or destroyed. You can only change its' state. Using electricity to "crack" water, takes more energy than what you get from the end product. Pound for pound, there is a lot more energy stored in petroleum that water. Many rockets are powered with kerosine.
I can see this as an enhancement to current cumbustion engines, by re-using wasted energy by converting to electricity and using that electricity to crack the water.
Interesting idea, though! The cool part is, that the by-product of burning an oxygen and hydrogen mix is water!
IMO.