The reason why water is banned bc, someone could pour an explosive liquid that can't be detected at all, and then be used as a weapon in a terrorist attack.
In 2006, a terrorist attack got prevented, where an explosive substance was put inside a bottle was supposed to be blown up, destroying a passenger plane.
I've Never Been On A Plane So I Actually Hace Never Faced This Sacrilege Before. Though Another Person Had A Good Point On People Being Able To Hide Dangerous Chemicals In Bottles.