Fact: We could run cars on water, but the oil companies keep it a secret
Untrue. Of itself, water has no energy left to give, so the idea of putting water in the fuel tank of a car, along with some magic ingredient which oil companies know about, is merely an urban myth.
However, as a source of hydrogen gas, water has the potential to be a fuel of the future. Hydrogen is produced when steam reacts with biomass, or when electricity is passed through water, or even when sunlight falls on water containing the right catalyst, but in this last case only in tiny amounts.
Hydrogen gas generated from water can be used as a fuel for transport and vehicles have been produced which run on it.