>if of course we could reach low-carbon hydrogen production

The biggest culprit of all issues with using hydrogen. The easiest way of producing it isn't much better for the environment than directly burning hydrocarbons. And it's exactly the way big fuel corporations want it.

BTW while with jet fuel it's about twice as much water produced, how about common car fuels, like gasoline and diesel (OMG I hate diesel, ban this thing for good already), could you calculate it? I'm not very good with hydrocarbons.

Indeed, but don't forget more water emissions in the sky means more clouds. If your calculations are correct (which seems like it from my perspective) this means we will effectively double the water vapor emissions from aircraft only. And clouds have a greenhouse effect too, but besides that, they capture the infrared (thermal) energy in the atmosphere in one place and release it in another one, thus moving energy around the globe through weather systems. Additional clouds would mean more energy would be trapped and moved around. That's not as harmless as it may seem, these effects should definitely be researched ahead.

Do you need a link to a chemistry textbook?

Assuming we're burning 1000 grams of hydrogen and have complete combustion and a reaction that is 100% ideal (it won't be like that, but pretty close):

2H2+O2->2H20

With that, amount=mass in grams/Mr or Ar. Hence the amount of H2 would be 500 mol. Using a stoichiometric ratio amount of water is 500 mol.

Now mass = amount×Mr or Ar, hence 500×(2+16)=9000g of water from burning 1000g of hydrogen.

That's a common misconception. Clouds have a greenhouse effect too. They're just capturing the infrared (thermal) energy in the atmosphere in one place, and releasing it in another one, thus trapping and then moving energy around the globe through weather systems. Additional clouds would mean more energy would be trapped and moved around.

No, it's ain't a joke really. When you will burn 1 kilogram of hydrogen you will get 9 (yes, NINE) liters of water. Good luck capturing that. Imagine all this water in the winter on the highway. Or up in the stratosphere as clouds.

I'm an engineer, I'm not forgetting anything.

1. It's minuscule in comparison with what hydrogen engines will do. You will burn 1 kilogram of hydrogen and you will get 9 (yes, NINE) liters of water. It's chemistry.
2. Yes, but by using it in jet aviation we will introduce the all-new very effective way to move that water high in the stratosphere, thus creating a lot of new clouds.

It won't work this way with combustion engines of moving vehicles, you know.

Well yes, but imagine all of the jet aviation to switch to hydrogen engines. I doubt the sheer amounts of water vapor introduced into the stratosphere as a result of that will do any better than current amounts of CO2 do. The greenhouse effect could even be worse. The effects of water vapor emissions should really be researched and compared to effects of CO2 emissions before we will put too much time and effort in hydrogen tech.