In coastal and arid regions experiencing severe water stress, seawater desalination serves as a critical infrastructure for freshwater supply. However, its high energy demand, particularly in hot and saline coastal regions such as the Persian Gulf, requires continuous energy and environmental monitoring. This study presents a scenario-based screening framework for evaluating the life-cycle global warming potential (GWP) and energy-related performance of desalination options for Bandar Lengeh, structured in accordance with the principles of ISO 14040 and ISO 14044. Using the production of 1 m3 of potable freshwater as the functional unit, a cradle-to-grave screening assessment was conducted based on regional operational parameters, energy statistics, and literature-derived inventory ranges. The screening confirms that under fossil-fuel-dominated power regimes, the operational stage accounts for the vast majority of carbon emissions (typically accounting for approximately 85–90% of total life-cycle emissions), making specific energy consumption (SEC) and grid carbon intensity the key determinants of global warming impact. Under the defined functional unit, system boundaries, energy conditions, and reference assumptions, the RO scenarios show a lower screening carbon footprint than the MED and MSF scenarios. Solar-based pathways and broader renewable-electricity scenarios, including wind-based low-carbon electricity assumptions, are estimated to reduce operational emissions under the defined assumptions; however, downstream life-cycle burdens, such as chemical consumption and equipment manufacturing, become proportionally more important. Finally, an operational monitoring matrix featuring actionable Key Performance Indicators (KPIs), baseline thresholds, and monitoring frequencies is proposed to support low-carbon operational decision-making in Bandar Lengeh and comparable Persian Gulf facilities.