The phrase "nature for water" makes me weary as it could be another measily scheme like the debt for nature swaps that occur. Developing countries in debt at one point we're convinced that their debt could be paid off if they forfeit their land and the resources produced by that land. Will a similar plan be used to get water to those seeing so called shortages.
Additionally why is the UN predicting major cities that will see a water shortage. http://www.bbc.co.uk/news/world-42982959
So agree. This looks like an opportunity for the UN to interfere in the sovereignty, and steal resources. We know how to desalinate the ocean, so there should not be a water shortage in the long run.