Adjusting prices in the way you describe is really difficult to do without causing some other problem.
Prices emerge from all interactions between buyers and sellers in a market for a particular good or service. The price contains information about the supply and demand of that good or service that is not obtainable in any other way.
When the government messes with prices, people stop making decisions based upon how scarce something actually is and instead make decisions based on how scarce the government imagines how scarce the thing ought to be. The resulting changes in behavior are really, really hard to predict in advance.
You used water as an example...
If a single shower starts costing you $1, you'd surely be willing to invest in a more efficient showerhead completely voluntarily.
Not necessarily at all. $1 shower would require very expensive water. At that price, I might forgo showering entirely. Maybe I'll be concerned if the price of water is that high, that it might not be available when I really need it, so I'll start stockpiling jugs of water in my basement, and end up using more water than I would have taking a shower at the market price. I dunno because it's hard to imagine and our imagination cannot be based on reality, because the act of taxing in this way is a deliberate decision to ignore reality.
The market for many commodities also extends beyond the boundaries of a single polity. Suppose in Country A, the price of a shower is $1, but in Country B, the price is the market price. Except, people in Country A may be using less water, so the supply of water will be greater than it otherwise would be, reducing the price in Country B, whose citizens would then act as though water more plentiful than it would be.
This is hard to figure out and plan. It is easier to tell people to use less and institute social stigma than it is to pick a socially optimal price that will do that just right.