Maritime decarbonization has become technology-led yet continues to underperform in real-world deployment. The missing link is not propulsion innovation but a lack of understanding of when, where and how much energy vessels actually require during operation. This presentation draws on PhD research into inland and short sea shipping to demonstrate how vessel behavior, duty cycles and environmental context fundamentally shape energy demand. Using real operational data, it shows why static assumptions and fixed infrastructure are misaligned with reality. The talk argues for a shift toward data-driven, vessel-centric energy-demand mapping as the foundation for effective electrification, hybridization and alternative fuel strategies.