Global environmental change can disproportionately impact vulnerable populations in informal settlements already struggling with diminished access to resources, conditions of poverty, and other inequalities. Simultaneously, climate variability is projected to increase global water scarcity and make "formal access" to water (i.e., through gray infrastructure sponsored by a centralized government) not only politically unlikely but also physically unfeasible. Cities will need alternative ways of delivering water to informal settlements that are reliable, sufficient, affordable, environmentally efficient, and fair. Using data from two informal settlements in the Xochimilco Municipality (Mexico City), we explore current informal arrangements for water access and delivery, and what roles are played by governments, water truck drivers, and residents. We found that self-organization through collective agency and community leadership were key for effective water delivery through private or public water trucks (pipas). One community showed stable leadership and strong collective agency, resulting in more "efficient" public water delivery and low levels of consumption of water from private sellers. In the other community, collective agency was hindered by lack of clear leadership and self-organization, causing residents to resort to individual action (i.e., buying more private water) rather than collectively organizing to gain sufficient public access to water. Our findings suggest that collective agency enables a positive feedback loop between water truck drivers and residents which allows efficient distribution with minimum public investment from the municipality. We argue that the explicit acknowledgement of the role of collective agency and its adequate compensation to create new agreements would create opportunities for more sustainable alternatives of water delivery in communities trapped in informal regimes of water distribution.