<p>Understanding the evolution of regional water use structures and their underlying drivers is essential for optimizing water resource management and promoting sustainable development at the regional level. This study investigates the three major urban agglomerations in China—Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD)—and analyzes data from 2014 to 2023 using information entropy, Lorenz curves, Gini coefficients, and grey relational analysis to examine the evolution of water use structure and its driving factors. The findings reveal that: (1) total water use remained stable in BTH, fluctuated upward in YRD, and slightly declined in PRD; (2) unconventional water resources (UWR) expanded substantially across all three regions, contributing to structural optimization, with agricultural and industrial water shares declining while domestic and ecological shares increased; (3) the grey relational analysis identified distinct dominant drivers across sectors and regions. In agriculture, the effective irrigated area and sown area for grain crops were the most influential; for industrial water use, wastewater discharge was predominant in BTH, the number of enterprises in YRD, and the proportion of the secondary industry in PRD; for domestic water, urbanization was the primary driver in BTH, per capita usage in YRD, and population growth in PRD; finally, for ecological water, UWR and reclaimed water were significant in BTH and PRD, but less so in YRD. These findings highlight the role of UWR in alleviating water scarcity and underscore the need for region–specific policies, as well as a more refined quantification of policy intensity in the future.</p>