You're not wrong, but it depends on where you are.
My wastewater goes back into the stream that my water came out of, so no net loss of water from the stream.
If you live in an arid region (and other places), you might be pulling out of an aquifer that is not being recharged at the same rate, thus depleting that aquifer.
No matter where it comes from and where it's going, it takes energy to treat and/or transport it to you, so you are wasting energy when you waste water. That energy is pretty well reflected in your water bill most of the time and is probably not a big part of your energy footprint overall.
I teach and do research in water treatment and water quality and it does bug me a little when people act like water is lost forever once it's used. There was this stupid commercial a couple months ago that implied that you using water in the US somehow deprived people in developing countries from getting water. Providing clean water in developing communities has been my passion for 30 years, my hobby, and now my job, but wow was that commercial stupid.
But there's no reason to waste water either.