The concept that water is so scarce that it's even rare for drinking never gelled for me.

But I still enjoyed the book.

I don't see it as a matter of absolute scarcity. In the book, drinking water for the poor is scarce, but the upper middle class has access to fountains and gardens, etc. If you think of it as a matter of control, it makes more sense. If you're on the inside of the arcology, everything you need is provided for. If you're on the outside, you're on your own. Resources are kept artificially scarce for any number of reasons, but mostly to support the status quo.