Hydrogen is abundantly available both on earth and in space, yet de facto mainly extractable from water.

Hydrogen is reconverted into water once it has reacted with oxygen. You can keep doing the process of splitting (in an electrolyser) and oxidation (by combustion or by an electro-chemical reaction in a fuel cell) indefinitely. Even the water from the ocean can be used to make hydrogen. So there is an abundance of water available. And after reaction, the water will simply fall from the sky as rain, after which it can be converted into hydrogen again via an electrolyser with green electricity. This makes hydrogen a sustainable and scalable molecule for energy-usage that does not need any carbon involved.