On this planet the act of "saving water" as such is very rare. What we usually mean is that we want to "conserve" water in a useful state. Ask yourself what we normally mean by the term: "saving water". You don't go down to the beach or sewer and grab some water to hoard in a bank, do you? Why not? It is over 95% water, isn't it? Practically pure.
Sure, but with all the unwanted stuff in it (usually stuff that in some contexts would be valuable) we cannot use the water for drink, irrigation, chemical purposes, or washing. And to remove the crud often is not practical, either in time for our immediate needs, or never, at any rate not without a greater expenditure of energy than we can afford for the purpose for which we need that water etc. etc. etc...
"Clean" water, meaning water that contains little enough of particular impurities for us to use it for particular purposes, represents low local entropy. Reducing entropy is expensive practically by definition, even when it is practicable at all. Avoiding the cost in entropy that we must pay if we are to turn contaminated water into clean water, is what we usually mean when we speak of saving water. Of course, there are other examples, such as saving ("conserving", being parsimonious with) the water we carried with us into a desert, where there is too little water, or out to sea, where there is too little salt-free water, or into polar regions where there is too little liquid water, but in terms of entropy and costs in energy and material, all such conservation problems are closely related.
Where the water is, relative to where we want it, is also important (for instance in Greenland there is more water than we want; we would prefer to have a lot of that water in the Sahara, but transport is too expensive for most practical purposes; this is another example of a cost in entropy) The state of the water matters too, eg if you happen to be in Greenland, dealing with too much ice is a problem when liquid is what you want; in the Sahara large quantities of ice might be very valuable and we would want to "save" them, protect them from heat. For example, they might be valuable because in the process of melting the ice, we could use the fact that in melting, it takes up so much heat that it condenses condense water from what seems to us to be dry air.
Incidentally, since you and your students obviously do think about non-obvious questions, here is another question concerning the eternal nature of water molecules: people who wish to illustrate how eternal molecules are, are fond of explaining that some of the molecules in every glass of water we drink had passed through or over the lips, lungs, or bladder of Cleopatra.
That is a nice exercise in arithmetic, but they are wrong of course. It is hardly possible even to state any such question meaningfully.
Think about some of the reasons and tell us some of your conclusions (anybody!).
Cheers,
Jon
