When you burn hydrogen and oxygen the product is water energy so the water is not lost.

I did realise that...

What I meant was that a "chunk" of water would be unavailable for watering duties while it was being held in Hydrogen form ready to be put into the vehicles. Whilst individual molecules will go round and round this (and the classic evaporation, cloud, precipitation water cycle), at ony one time an approximately constant quantity would be in the Hydrogen storage phase. A bit like large amounts of water being held in clouds, so it's not in the oceans/rivers etc.

The second point was that just as the clouds redistribute water around the globe, so will using it as fuel. I was wondering whether this would be large enough to have an effect on the world's climate.

On the other hand, if the glaciers are melting and we're expecting sea level to rise, then maybe by taking large amounts of water out of the classic water cycle we could alleviate or at least mitigate its effects.

"On the other hand, if the glaciers are melting and we're expecting sea level to rise, then maybe by taking large amounts of water out of the classic water cycle we could alleviate or at least mitigate its effects."

Hahaha, very good point! I certainly don't have the information to do the calculations but I did find this graphic illustration, you may be on to something. The small blue sphere on the left represents volume of water vs earth and the one on the right is volume of atmosphere:

Kind of frightening when you see by how small a margin we subsist.

No graphics

Shows pretty here. Try this: Comparison

Plenty of water in the outer solar system- we can ship it back to Earth in 10,000 ton chunks. Once we have the supply chain set up, we can ship water to the Moon and Mars also. Theoretically, we humans can survive anyplace there is sunlight and water.