Sorry but H2O is not a basic human right as defined by the UN, you have a right to a standard of living that mentions food, the cost of that standard of living is a personal choice not a right.
Access to water may not be a basic human right in the original UN Declaration of Human Rights, however the right to food, sanitation and water for essential human health has been adopted as a human right via a resolution which was passed by the Assembly in July 2010 (although it had been instituted in other conventions earlier, thus essentially ingratiating itself as a human right and into the national laws of relevant signatories if not formally then de facto).
However, this is all irrelevant. Even if the OP could not get water on a plane other than by purchasing it, that is not a deprivation of a right
per se.
The quality of water I would assume from galley dispensers must be potable in some shape or form, otherwise I don't see why someone can't be taking samples of that water, testing it and making a generous dollar out of the airlines for serving water which is less than desirable quality (i.e. affecting human health). In saying that, I think the water in the galleys (viz. used) is just the tap water from the local area (assuming that the tap water is drinkable), because if I have a cup of tea on board from some locations I can taste things like chlorine. This is, of course, notwithstanding the quality of water fittings on the aircraft (i.e. pipes, containers, spouts, etc.).
All of this still doesn't quite get to the point as to why the OP couldn't be offered a humble cup of water from a galley dispenser (which supposedly has a source of water and is water that is drinkable)?
Now that I re-read the OP, when "tap water" was said did the FAs think that the basin tap water in the bathrooms was implied, and therefore said the water is undrinkable? They would probably be right, but they probably then didn't think that "tap water" as said by the OP was supposed to be the water they would dispense in the galley e.g. for making water-based hot beverages.