Who are you referring to when you say "deny access to water"? Because if you are talking individuals, congratulations, you are actively calling for the death of individuals.

Water may be scarce, but the amount available still is much more than enough for living beings.

The main difference is the isn't any speculation involved. No one have the financial incentive to create artificial scarcity. This is a major advantage when talking about the most basic of life needs.

When you buy bottled water you're buying water by volume at a price set by the company that manufactured it, pre-packaged. They set the price based on the costs they incur to bottle it - beyond what you would pay to get the same water as a civilian, in part because creating shelf-stable bottled water is more difficult and expensive than producing regular tap water. To get water, that factory is paying for delivery and treatment infrastructure and ongoing maintenance costs for their equipment. Your water bill similarly covers those ongoing costs. If you buy a pack of bottled water and it leaks, nobody from the bottling company is going to come patch that hole and refill the bottle.

This is almost the same as the distinction between paying your electricity bill and buying a pre-charged battery.

As an ordinary civilian I can't buy a year's worth of electricity and stack it up in the corner of my house (some sort of enormous tesla powerwall aside), and I can't buy a year's worth of tap water and store it in a giant tank in my bedroom. Doing that sort of thing would require a bunch of infrastructure that I don't own. Instead, I pay monthly to have both electricity and water available on demand, and the cost is based on how much I use. Long-term storage like that also would introduce additional problems, like the passive discharge of batteries or the need to store water in special containers that won't leach chemicals into it and won't allow the growth of naturally occurring algae.

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.