Yeah uh, water is at least fairly easy to reuse and isn't "wasted". My old apartment recycled water on site (one of the first in north america to do it) for the toilets/landscaping. If cities would just put in non potable water lines and make it code for new buildings to use it for toilets and landscaping we'd really reduce how much drinking water is used.
im not sure how you and every single other person who responded to my comment came to think that this was about saving resources. the only reason i dont want to use water is because water is a terrible way of taking care of waste on a mobile platform. water is super heavy and you have to use a lot of it each time you flush. and on top of that, the result is a toilet that slowly gets super dirty. plastic is better because you could store hundreds of "flushes" in a small box that doesnt weigh anything and because the toilet is totally and completely clean after every flush assuming all the waste stays inside the bowl and doesnt splatter up and out all over the place (which is a pretty good assumption). and lastly the waste can be stored with almost the minimum possible weight overhead and doesnt make its storage tank dirty or smelly. overall you have a lightweight, highly clean and maintenance free system.
in the end, water and plastic are largely the same thing. oxygen, hydrogen and carbon are all you need to make either. recycling plastic will, in the future, be trivial. same with water and all other substances.