Water is one of the most precious resources in the world, and the only reason it does not have wars fought over it in a similar way to oil is due to the reason that it is relatively abundant even in areas that are technically water scarce at this current point in time. Suffice it to say that whenever the water table dips below sustainable levels, everyone will suddenly realize just how crucial this sort of thing was for them without a shadow of a doubt.
Hence, your main priority throughout the remainder of your life on this planet should be to reduce your consumption of water as much as you feasibly can. That means that you need to figure out if trying to power wash your car would make you use water in excessive quantities, and let us be the first to tell you that it is actually superior to garden hoses in that respect. The thing about using a pressure washer is that while the gallons per minute are far greater than what hoses put out, you need to use less water because each droplet hits your car with such force that it has the impact of a hundred droplets that might come from a hose.
As a result of the fact that this is the case, you can safely use a pressure washer to clean out your car without concerning yourself with the impact that this might have on groundwater supplies and the like. People that worry about such things should now that pressure washing is one of the most effective ways to use water as of right now.