There are two main sources of the elements needed to do so on an industrial scale. Water, and hydrocarbons/the air.

The two main exhaust products from burning oil are carbon oxides (CO2 and CO) and water. Burning or otherwise reacting oil and collecting the water would probably be the cheapest way to create water on an industrial scale other than to just collect it from anywhere it's plentiful.

Water scarcity isn't a literal scarcity in the sense there isn't enough, it's a scarcity in the sense it's too expensive some places for poor people to afford enough clean water.