Access to clean and safe water is essential for human health and well-being, but recent and substantial increases in the cost of water for residential customers in the United States endanger the health of those who cannot afford to pay. This study identifies pathways through which unaffordable water bills may influence the behaviors and health of vulnerable people. We interviewed a sample of low-income residential water customers who were experiencing water bill hardship in Boston and Chelsea, Massachusetts, U.S. between October 2018 and December 2019. We conducted a thematic content analysis of interview transcripts. Results showed that some participants improvised ways to pay their water bills, and some confronted obstacles that made it simply impossible to pay at times. Behavioral responses to coping with high water bills were influenced by household earning potential, self-reported health status, caretaking responsibilities, and accessibility of utility assistance programs. Consequences of unaffordable household bills included reduced access to other necessities, debt accumulation, risk of water shutoff, housing insecurity, and public humiliation. Reported health-related impacts of water bill hardship were food insecurity, underutilization of healthcare and medications, and decline in mental health. Comprehensive reforms at all levels of government are needed to make water affordable for all low-income households. Federal investments in water infrastructure, state oversight of affordability and human rights, as well as municipal tiered water pricing and comprehensive assistance policies for low-income households are needed to address the growing water affordability crisis and to mitigate harm to the well-being of vulnerable residents and communities in the United States.