Btw, I say all of this as someone who's (basically) unemployed and receiving maximum food stamp benefits. I don't honestly think this particular issue is any kind of "poor" issue. It shouldn't be taxed anyway, but it's not some nourishing bottle of human health. It's pop (soda), it shouldn't be your main source of fluids.
I agree that all tapwater should be of higher quality than it is in most places (I live in Portland, OR, so I'm kinda spoiled for public water and I'm fully aware, so I won't tell people tapwater is the end all solution). If you're going to tax every source of liquid nourishment, then your water should be flowing perfectly drinkable and downright tasteless.
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Viviff on the go! (or from the toilet)
http://images4.wikia.nocookie.net/board8/images/8/8b/ViviffChampS1.jpg