The thing is, in the usual course of circumstances, why would you drink tap water from a plane or any form of public transport anyways?
Any drink that you receive from a galley water source is usually boiled, thus any pathogens are going to be killed, so it's unlikely you would catch anything.
Obviously on a premium flight you can ask for water, poured from a bottle whenever you like. If you can't afford to buy one on an economy flight, then perhaps the reason they aren't allowed to give you tap water is because it hasn't been boiled.
In many ways though, I feel you only have yourself to blame if you can't buy one.
WHSmiths at UK airports now does a deal where you get a large bottle of water and a free newspaper for like a £1 and they sell thousands of them. I personally always buy a bottle of water before boarding, even if I'm flying premium.
It's silly to fly without any means of a way to pay for something on a budget flight where they charge for such items and everyone is well aware that it's the case.
I'm sure though, that if you were clearly dehydrated and in urgent need of water that they would readily give you a bottle of water for free if required.