Background: Unsafe drinking water remains a pervasive environmental hazard, exposing nearly half the world's population to preventable health risks and exacting substantial economic and social costs. Despite global progress under Sustainable Development Goal 6, inequities persist, and emerging threats are reshaping the risk landscape. Methods: This narrative review synthesises recent evidence (2019–2024) from PubMed/PMC-indexed literature, supplemented by World Health Organisation and World Bank reports, to quantify the public health costs of inadequate drinking water. Studies addressing microbial and chemical contamination, quantified health outcomes, and economic burden were prioritised. Key findings: Diarrheal disease from microbial contamination causes an estimated 1.4 million deaths annually, with children under five disproportionately affected. Chemical contaminants, arsenic, fluoride, and nitrates, affect hundreds of millions and produce long-term disability through cancer, cardiovascular disease, and neurodevelopmental harm. The economic burden in low- and middle-income countries exceeds US$100 billion annually in treatment costs and productivity losses, with women, rural communities, and marginalised populations bearing disproportionate costs. Climate change and emerging contaminants, including per- and polyfluoroalkyl substances (PFAS) and microplastics, are intensifying risks. Conclusions: Investments in safe drinking water yield among the highest returns in public health, averting mortality, reducing healthcare expenditures, and advancing equity. Failure to prioritise water safety undermines global development goals and deepens health disparities. Coordinated action across health, water, climate, and finance sectors is urgently required.