Electrolysis would do it, but as you say, its inefficient with water so it could never work well enough to power a car.
But think, pure oxygen makes stuff burn very quick.
Hydrogen is the most volatile/explosive/reactive substance around.
Imagine hydrogen going off in pure oxygen...
You'd get a scooter engine more powerful than a car.
The plus side is, hydrogen + oxygen = water, so when you split the water, then ignite the products, you get more water, which is theoretically an infinite fuel source.
Haha, you're a clever one!
Yeah, it would be the same with the water if you could split it. You'd only have to top it up every once in a while
Some electric cars are pretty quick nowadays.