Human population density doesn't need to adversly affect water supply. Hum,an political and social organization of a maladaptive structure deplete and degrade water supply, and that is potentially subject to reform.
Human poltiical organization historically is badly formed with the most powerful generally intimidating others in to submission. One might romaticize about some Rousseuian state of happy savage natural society, and miss the point entirely of the actual historical human state of affairs of government through an occams razor of social neccessity of having power with the least work done otherwise. The phrase 'good enough for governent work' is approriate enough for the object of politicians is generally to attain power rather than to reform society for the actual, empirical good. Peter the Great attempted reforms as a Tsar and was to some degree successful-yet authoritarian governents with wrong premises and implentation accomplish by far the most social repression and it is unreasonable to speculate than an particular Earthly Government presently has the right answers about how to conserve and restore planetary water quality. Knowledge isn't lacking in some quarters of academia, it simply isn't applied politically within a just, democratic basis.
Democracy has a tendency to permit anarchic social development and that was especially noticably in the American pioneering era. One might just 'go west' and find sparesly populated areas requiring little regulation. Science and technology were co-evolving and were not much suitable for keeping large areas of the ecosystem intact either. In today's political environment conservationists may fight for 100 foot or 200 foot green buffers along streams with industry that prefer to log to extinction. In poor countries the people wash clothing in water supplies and industry may pollute groundwater and surface supplies. Few nations distribute sloar stills for water starved people along coastal areas that are made of metal and can produce a couple of gallons of fresh water from salt water each day.
Human lifestyles require a long range renewable ecological environment so far as is possible within the laws of thermodynamics. National boundaries and infrastructuries must be designed on an ecologically strong foundation such that security in social and ecological environments are omnipresent. Humanity can become a friend to itself preserving in conserving water supplies or stock fund resources [such as were detailed in 'Ecological Economics' by Herman Daley (an ecological economics introductory textbook)]. Humanity can learn to work for the well being of the planet instead of regarding it as an inexhaustable practico-inert object until it is over-used and broken.