No, it's not :)

Where would you get the energy to split the water? Why not just use that energy directly :).

The idea is to use the water as an energy carrier. You split the water into H2, than use that to power a car producing water as the only output.

>Where would you get the energy to split the water?

From a battery.

Watch a YouTube of a hydrogen car review. They produce maybe a cup of water for a consider amount of driving. It’d evaporate relatively quickly.

They already produce plenty of water in the exhaust.

Right, I did a simple calculation, for the same distance driven, both gasonine and fuel cell cars produce pretty much the same amount of water, about 100ml/km. Only difference is, with gasoline that water is mostly vapour, so it doesn't get on the road so much, fuel cell cars have small tanks not to wet the road constantly.

How would that work? That sounds like a perpetual motion machine.

Like how a gun works, essentially a battery for the initial motion, using the subsequent motion storing some for the next initial motion.

Guns work by having chemical explosives in them. Which is also how gasoline powered cars work.

They definitely do not work by using a depleted battery to charge a second battery to create energy out of nothing.

gasoline cars also use a battery to initiate the engine.

Yes. However they don't use it to both power the car and charge the battery. Kickstarting the engine with a battery is very different from only having a battery and also using that battery to charge itself and the car at the same time.

The original poster was proposing a closed system where you would turn water into hydrogen (a very energy intensive process) and then turn it back into water (an energy releasing process) inside the car to provide all the energy. This is not how physics works. You cannot use your energy source to create more of your energy source without an external energy source. It just doesn't work that way.