This deliverable [D4] provides a critical analysis of the FuelEU Maritime Regulation (EU, 2023a) and its alignment with the broader European policy architecture for maritime decarbonisation. The report addresses the interplay between FuelEU Maritime and related policy instruments, including the REPowerEU Plan, the extension of the EU Emissions Trading System (ETS) maritime transport, the Alternative Fuels Infrastructure Regulation (AFIR), ReFuelEU Aviation, and the Renewable Energy Directive recast (RED III), to identify policy synergies and frictions affecting the energy transition. The analysis is part of the POTENT-X (Ports as Energy Transition Hubs) project under the Clean Energy Transition Partnership and coordinated by Chalmers University of Technology. It draws on policy document analysis, comparative assessment across seven countries in the North Sea and Baltic Sea regions, regional data collection, and input from the Living Lab Networks operating across both regions. Three structural outcomes are outlined in the report: EU’s maritime decarbonisation framework is structurally coherent: carbon pricing under the EU Emission Trading System (ETS) provides a cost signal; fuel standards under FuelEU Maritime mandate fuel-switching; the Alternative Fuels Infrastructure Regulation (AFIR) establishes port readiness requirements; and Renewable Energy Directive recast (RED III) defines a sustainability criterion for renewable fuels. However, partial harmonisation across these instruments introduces compliance complexity. Hydrogen supply gap represents the binding constraint on e-fuel availability for maritime transport. Against the Hydrogen Strategy target of 40 GW of electrolyser capacity by 2030, installed EU capacity was approximately 308 MW by 2024, representing less than 1% of the target. The trajectory makes the REPowerEU electrolyser ambition questionable. Without significant changes in deployment, near-term FuelEU compliance will be carried by biofuels and LNG rather than RFNBOs, despite the regulatory emphasis on e-fuels. Besides, competition with the aviation sector, driven by the ReFuelEU Aviation synthetic fuel sub-mandate, may further constrain available volumes. Regional asymmetries result in diverse transition pathways: Denmark and Sweden benefit from low grid carbon intensities and advanced Power-to-X strategies, whereas Poland faces higher electricity prices and substantially higher grid carbon intensity than the Nordic average. Grid capacity at ports represents an additional bottleneck, as simultaneous demand from onshore power supply (OPS) rollout and Power-to-X production increases electricity requirements. This asymmetry implies that compliance costs will diverge significantly between operators bunkering in Nordic ports and those in higher-cost regions.