See, there is already a lot of stuff to worry about in life. If you can act on a hypothetical problem, why not redirect your effort to an actual one for some real world benefits?
Should I be insulating my house to save energy even if I lived in a warm country? See, this sound ridiculous but so is saving water when there is no benefit from it.
I'll bite - it doesn't sound ridiculous because it is highly likely that there is going to be power hungry air conditioning equipment in use. I think it's an example of an unfortunate phenomenon where if you deviate from the accepted cargo cult practices you run the risk of missing some detail and making things worse, at which point you get the blame, but conversely you don't get the credit for doing things better than average. In fact it's likely there is a peanut gallery just waiting for you to make a mistake so they can punish you for being a renegade.
The GP saving water would have no impact at all on the people that miss it. It's not a "do your small part", it's simply no part at all.
In the past, tradition was one of ways to pass knowledge. There was no youtube or google available and people were generally were not able to read or attend classes.
Even today you teach your kids certain things they just have to do before they can have knowledge of why. For example brush their teeth.
Even then, repeating things like that is still one of the ways we cope with complexity of the world around you.
I am certain you are probably repeating some instructions from somebody on how to do something just because there is no time in the world for you to evaluate every single simple thing and decision.
On the other hand if somebody starts writing about it and discuss with other people, I expect them to put at least a little bit of effort into thinking about it if you are equipped for it (or shut up if you are not).
Repeating mindlessly on the Internet things you learned from somebody else is one way the world is broken today.
Of course there would be consequences if you were rude. Just not to you.
For some reason people conflate the two and then it is difficult to have a civil discussion on the Internet.
Don't agree with somebody's response? Don't try to shift the discussion to something else like the particular choice of words and whether you "feel" it was rude. If you can't or are not willing to discuss the actual arguments it is better to stay silent.
The whole point of recommending people to save water isn’t that one’s own water conservation will directly cause a measurable re-allocation of water to someone else. It’s more akin to moral advice, something like “act in a way that would have good outcomes if everyone acted that way.” I suppose it’s okay to explicitly dismiss all moral recommendations of that form, but it’s worth noting that this form is extremely common and not something I would call “strange.” It’s basically the same moral recommendation as “don’t butt in line even if no one is going to stop you.”
To map that idea directly onto your situation, I guess it would be something like “if 10 million people moved to the vicinity of your small river, there probably would be a water shortage that would require everyone to be conscientious about their water usage.”