Water injection has been known as a way to increase engine power at least since WWII, when it was used to give an added boost to supercharged aircraft engines . . . though the major mechanism was to forestall detonation.
As for the claims made in the linked ad . . . when it comes to increasing fuel economy by 2/3, I believe the product has every bit as much validity as adding cow magnets to your gas line or putting a scale model of the Great Pyramid of Cheops over your intake manifold.