The energy has to come from somewhere. If you put in water, and you get out water and energy, you're violating all sorts of fundamental laws of physics.

His engine does work, in a sense. That's why he was able to patent it (perpetual motion machines are summarily dismissed by the US patent office these days). But the energy doesn't come from the water, it comes from the "12V DC" that's barely mentioned but clearly in the diagram. The 12V DC electrolyzes the water (splits it into H_2 and O_2), and then the engine burns these back into water.

But the energy you get out of burning the H_2 and O_2 is always less than the energy you had to put in to split them in the first place.

This dude has simply built a very complicated, inefficient, electric car.