The credits aren’t dollars of water. They’re kilograms of water. Every individual gets an equal allocation (per month or year) at no charge. If you get more than you need then you sell it at market rate, maybe to that corporation trying to grow corn in the desert. Chances are that as a result, their water costs will be infinitely more than a given household. At the end of the day, no more water will have been taken from the reservoirs and aquifers than is sustainable because only so many kilograms of water were rationed.