Sanitizing with StarSan in a spray bottle uses much, much less than filling a vessel - even if you only fill the vessel half way. I can usually make it through an entire brew day with less than 16 oz of StarSan this way. StarSan is a contact sanitizer - only the surface to be sanitized needs to be wetted. The rest of the sanitizer filling the vessel is unnecessary. Of course, if you save the StarSan to reuse on another brew day this is a moot point.
Like others, I save leftover hot water from the HLT for cleaning, and use water from cooling for cleaning/rinsing. Cooling water also gets put into empty 1-gallon water jugs and frozen for use when recirculating ice water to chill lagers. Remaining chilling water gets used to water plants in the summer, and to fill the washing machine in winter. I let my mash sit a while after sparging and collect the last of the low-gravity runnings for starters. Even if those runnings are too low a gravity for a starter, it's that much less DME and water that I need to use.
I do these things, and I live less that five miles from Lake Erie - so there's really not a water shortage here. I think conservation is a good practice regardless of whether there's a shortage. I do things like turn off the water faucet while brushing my teeth. That might only save a little water, but it also saves all of the energy used to treat it and pump it to my house, as well as reducing the amount of chemicals that need to be used, produced and transported. It's remarkable how far a little forethought can go toward reducing and conserving the resources we use.