Kish Island, a strategic region in the Persian Gulf, currently relies on reverse osmosis desalination and fossil-fuel power plants for water and electricity supply. Given rising fuel prices and environmental concerns, this study presents a technical–economic evaluation of a hybrid renewable energy system integrating solar and wind resources to sustainably meet the water and electricity demands of 40,000 residents. The system utilizes RO brine as feedstock for a chlor-alkali electrolysis unit to co-produce valuable by-products: surplus electricity, chlorine gas, 50% liquid caustic soda, and hydrogen. Climate data was sourced from NCC Tide, PVsyst, and Wind Atlas. Results indicate that tidal energy is economically unviable due to low tidal range (~1.29 m). MATLAB-based optimization determined that installing 14 × 12 MW wind turbines and a 59.25 MW photovoltaic plant—without energy storage—is sufficient to fully cover annual demand. In a simplified economic analysis (excluding operational costs), the initial investment is estimated at 17.36 trillion tomans, with an annual profit of 5.85 trillion tomans, yielding a payback period of 30 months. When operational expenditures (salt, maintenance, etc.) are included, the net annual cash flow decreases to 4,837 billion tomans. Nevertheless, financial analysis using an 18% discount rate reveals a positive NPV of +8,537 billion tomans and an IRR of ~27.25%, confirming strong economic viability.