This runs into problems when you try to apply it to groups, and to second/third order effects.
This also, only works where both parties are able to negotiate, capable to negotiate, and are both good actors. Just like in a relationship, you need two people working on it. If one person does not, then all of this good intentions goes out the window.
Say, you have one bad actor. And you negotiate to share water.
When do you kill the other guy? Before, during or after he steals the water you need to live?
It is self defense after, but at that point, you are dying…
And, you will find that the only solution in most cases is to kill narcissists/sociopaths as soon as you recognize they are such. That kinda goes against the nice, simple rules stated above.
Sociopath and psychopath's psychology are naturally afraid of dying more than any good man who is willing to kill for survival, his family, etc. I say let them know you are very capable of doing so and they will most likely leave you alone. In other words grab your sword if you don't have one and learn to use it.
And then sharing half the water so we can both survive because being kind is important too.
It's only when the good man, because of his ignorance and lack of experience, gets into believing he needs to be disarmed and disadvantaged do those types of people tend to take advantage, because they then see the opportunity to act out their unfortunate psychology. My opinion.