Objective: To evaluate the association between intermittent water supply, Family Health Unit (FHU) coverage, and COVID-19 spread among older adults (≥70 years) in Bauru, São Paulo, and to provide insights for targeted public policies. Methods: This retrospective cohort study was conducted from March 2020 to July 2021 using secondary data. Variables included age (60–69, 70–79, 80–104), gender (female, male), FHU coverage (present, absent), water supply (continuous, intermittent), and geographic region (South, Center, East, North, West). Poisson regression models were used to estimate relative risks (RR). Findings: A total of 7,540 COVID-19 cases were recorded. The West region accounted for 42.45% of cases, while the South region had the lowest proportion (14.20%). Individuals aged 60–69 years comprised 41.34% of cases, followed by 70–79 (32.11%) and 80–104 (26.58%). Lack of FHU coverage was associated with 83.51% of cases. Intermittent water supply increased infection risk by 17.5% (RR = 1.18; 95% CI: 1.08–1.28). After adjustment, the Center region showed higher transmissibility (RR = 1.85; 95% CI: 1.54–2.22), whereas the West region had lower transmissibility (RR = 0.53; 95% CI: 0.47–0.60). Conclusions: Insufficient FHU coverage and intermittent water supply were key determinants of COVID-19 spread, particularly among older adults. These findings underscore the importance of strengthening urban infrastructure and ensuring consistent water access to mitigate public health challenges in medium-sized cities.