This is like claiming that paying your garbage bill or firefighting bill (in places that charge for it) is "trading" your sanitation or protection from having your house burn down. They're services, not products.
Even after defining what the difference is... I'd be interested in understanding what makes trading in "water-delivery-and-treatment-infrastructure-and-operational-costs" OK, but trading in "water" not OK
This is almost the same as the distinction between paying your electricity bill and buying a pre-charged battery.
For water I could certainly buy a bunch of bottled water and keep it in the kitchen, but how am I going to run that through my faucets to wash my hands or run it through my showerhead? You can certainly do all that - and people in areas with limited access to tap water have to - but it's not something the average person is going to do.
Where your tap water comes from is also a factor here. When I lived in a rural area growing up, our water was well water - there was literally no way for us to stock up a month worth of that in advance, because we relied on the natural return of water into the soil to ensure we had a supply to draw from in the well. Things like desalination also have limited throughput, so if you max that out the only way to get more water is to take it from someone else.