My city is in the middle of the worst drought in recorded history. My showers are typically under 2 minutes and I have to shower with a bucket to catch otherwise wasted water to use to flush the toilet. I also shut the water down when I am wet enough so I can scrub myself without having unneeded water flowing then start it back up to rinse.
Plus, water is damn expensive!
Who here really has the time to stand, think and waste in the shower?
In my city the water comes from underground too. The problem arises when there is no rain and cleared land produces more runoff than absorbtion.
Coupled with heavy use by people ground water levels are reduced. This not only affects us but trees and plants that rely on these water levels will die off.
However, as the other commenter mentioned, normal citizen use and its affect on this is negligible. It's when you have industrial water extraction that is the real problem.
I did the math for Socal the last major drought, and normal people using water was like 2-5% of the water usage. And that includes lawns and stuff. Farming was the vast majority of water usage.
The cost is that you deplete the aquifer. Generally speaking, water pumped out of the ground doesn't replenish (except on geologic time scales). That's what I meant by the fossil fuel comparison. It's not like taking water from a stream or a lake replenished by snowmelt. Once that aquifer is dry, it's dry, and the land becomes dead.