How do you make water? Is there a finite quantity?

For every action there is an equal and opposite reaction. We are able to disassemble water molecules by running an electrical current through it in order to separate oxygen and hydrogen bonds. The reverse ~ If oxygen and hydrogen combine then the reaction should give off an electrical discharge i.e. a lightning strike. Suppose every time we witness lightning water molecules are being formed in the earth's atmosphere.That is logical to think there is not a finite volume of water contained within the earth's physical boundaries (sky, surface, and subterranean).Now about all those flood theories where there isn't enough water to cover the entire surface? smh