An electrolyzer is used to separate the hydrogen from the oxygen in water. Doesn't matter if hydrogen is burned or used in a fuel cell, (which can can be described as close to rusting), it's by-product is water.

Hydrogen doesn't make much of a crater. It's much lighter than air and has a very short flash time. It tends to just float up very fast, as opposed to diesel and fertilizer. A solid storage form of hydrogen will be much safer than gasoline or propane when used in your vehicle.

Electrolyzer/fuel cells have the potential to be 70 to 90% efficient, with cycling of about 60%.

I've heard we have plenty of fossil fuels left, but it's going to price itself out of a market.