Pakistan faces a critical energy-water-climate nexus, characterized by chronic electricity shortages, freshwater scarcity, and climate vulnerabilities. This study presents the first comprehensive, site-specific techno-economic, environmental, and direct comparative analysis of Floating Photovoltaic (FPV) and Land-Based PV (LBPV) systems across four major hydropower reservoirs in Pakistan (Aza Khel, Darawat, Mana Storage, and Tanda). Utilizing high-resolution PVsyst simulations over a 25-year horizon with standardized 1 MWp configurations, we integrate performance modelling, financial analysis, and hydrological assessment. Results demonstrate that FPV systems consistently outperform LBPV, achieving 1.3–2.5% higher annual Performance Ratios and yielding 3.8–5.2% more energy (38–52 MWh/year per site). Economically, while FPV entails 8% higher initial capital expenditure, its superior energy yield and significant co-benefits result in competitive Levelized Cost of Electricity (LCOE) and stronger financial returns (e.g., IRR of 15.3–16.4% for FPV vs. 14.6–15.9% for LBPV). A critical environmental advantage is FPV's capacity to reduce reservoir evaporation by 4,885–8,141 m3 annually per 4,652 m2 coverage, directly enhancing water security. Cumulatively, each FPV plant avoids 11,632–15,760 tCO2 over its lifetime. This research establishes a replicable methodological framework for multi-criteria assessment of solar technologies. It provides robust, quantitative evidence that FPV is not merely an alternative but a multi-benefit solution for Pakistan, synergizing clean energy generation with sustainable water management and climate resilience. The findings offer actionable insights for national energy policy, infrastructure planning, and investments aimed at a low-carbon, water-secure future.