You have to heat water to get steam and that takes energy. And time to get the water hot.

Unlike the Hindenburg, modern vessels do not disenegrate. There has been extensive test run on hydrogen fuel cells. They shoot a very narrow beam of flame up and away from the ground. Also Hydrogen is so much lighter than gasoline vapors, Propane gas, and compressed Natural gas, which is already used in modern internal combustion engines. Hydrogen can also be stored in tanks that have been outfitted with Carbon Nanodes which are derrived from the incineration process of chicken feathers at a specific temp and pressure. This allows for higher volumes of hydrogen at lower pressures thannwould otherwise be needed so explosion hazzards are mitigated.

@Ceaselessmind i am sure with modern technology a Tiny amount of water could be kept quite hot & ready to go making the awful hardship of planning your day unnecessary.....
And if we used water, the tyranny of suppliers would be Ended, a Huge benefit to everyone but them......

@AnneWimsey To address your point, consider the amount of energy it takes to maintain a waterheater in a house. Easily one of the biggest consumers of electrical energy in the home. It does not even keep the water hot enough to pressurize the sytem to steam. In any sufficent amount to make a car practical that would be a large and heavy amount of water to lug around. Another fun fact, cars have been steam powered before. It was highly impractical then and it is even moreso now. The other side ot the equation is, how are we going to turn the water into steam to power a modern vehicle to perform and meet the demands of the average automobile user? If through batteries, then you need a substantial amount of them to produce the current needed to start the process and/or, as you suggest, keep a "tiny amount of water could be kept quite hot and ready to go" would take a tremendous amount of energy. That means it is then therefore more economical to just have an electric car. If by any other energy source then you would need a fuel of some sort to supply the heat needed to generate that said energy to make steam. It again is not practical.

Secondly, a steam engine is far more hazzardous than hydrogen fuel cells. Often times safty valves fail and the tanks explode. This is why tractors and steam locomotives fell out of favor. Sure, you can go out and cut some wood or get some coal and make a fire, add water and produce steam to power them but, if everyone did that, then a huge problem of deforestation would arise. Coal is much worse than the fuels we currently use not to mention the mining effects on the landscape or the toxic waste the ash leaves behind. Many of which are highly carcenagenic.

In summary I fully believe the least impactful, and most benefical path would be in Hydrogen fuel cells adapted to our current automotive fleet, or perhaps the salt battery that has been developed in Germany, I believe, here recently where a bag of powder is added to water and it makes a battery electrolite that js nonharmful to the environment and equates to about 11 cents a mile in economy with most test vehicles averaging 400 to 500 miles on a single tank. These are innovations that can replace fossile fuels and minimise the consumer impacts while making travel safer and cleaner.

@Ceaselessmind ummm, a Stanley Steamer set the world record for land speed, only broken in 2009 by another steam-powered vehicle.
I use propane (another well-tested/trusted technology) to heat my water, it's almost as fast as those "point of use" water heaters (which are 3-5 times the price but also proven technology)
Your technology analogies are SO outdated!

@AnneWimsey seriously? Since when was a one off type automobile a sustainable modle for mass production and consumer safety? You can do much better than that. Sure, the "tankless" water heater is a device that has been implemented in the home. It is NOT suitable for every home and the require maintenance. Having been a plumber I have installed a good many. However they do NOT make steam or pressure. If you seriously think any job blow can cobble one into a car and run with it you are sadly mistaken. Those devices are desinged to handle normal household pressure values. To power a vehicle you need many many times that to drive a turbine or an engine. In order to be compact enough higher pressure is needed to produce the amount of force on the rotating assembly to move the average motorist to navigate the hills, curves, and. Stop and go driving. Still not a practicle technology. Your home water heater is NOT a reasonable basis to usurp the internal combustion engine. It simply is not going to happen. Besides, if you have to use propane to power your car may as well have the engine converted to use propane and save yourself the extra energy loss in trying to heat water to a steam pressure and modify or replace the engine. Again not cost effective. Further more you are still using... PROPANE... A fossil fuel controlled by big oil.... So it is self defeating.

@Ceaselessmind ummmm, a "one-off vehicle? Not until tne gas & oil interests drove them out with lies....how many actually hurt someone, even with the technology of that time?
Oh, and i can tell you from personal experiencw that that steam powered you so denigrate is what powers nuclear submarines, among other things...what do you think tne nuclear reactor makes, besides heat.......
Oh, and propane from Natural Gas is 99% efficient, and this country & Canada have Huge reserves!