In 2017, Hurricane María left more than a third of Puerto Rican households without water services. Cascading failures—including the simultaneous collapse of water, electricity, and transportation sectors—presented serious challenges to the timely restoration of governmental services. In response, families across Puerto Rico adopted self-organized coping strategies to obtain the basic resources they needed, including safe and sufficient water. Drawing on the fast-growing literature on household water sharing, we examine how Puerto Rican families shared water as a response to disaster. Using participant-observation data, interviews, and social network data, we studied water-sharing networks in three municipalities—urban, peri-urban, and rural—in western Puerto Rico in the aftermath of Hurricane María. We found that extensive water sharing (in 85% of households) spontaneously emerged in the wake of disaster, in previously water-secure rural, peri-urban, and urban communities. Households relied primarily on kin and neighbors, and women had more extensive sharing networks than men. Water-sharing arrangements were typically a form of generalized reciprocity, with little expectation of direct payback. We conclude that water-sharing networks are an important—but understudied and underutilized—component of disaster response. Our research indicates that water sharing should be more explicitly planned for and included in disaster preparedness plans. If water sharing is the dominant approach for coping with disaster-induced water insecurity, we argue, it must be at the core of disaster response.