Closing energy efficiency gaps in shipping to support climate goals requires understanding seafarers' operations during energy-efficient route planning (EERP) and their preferences for assistive technology. We developed a Hierarchical Task Analysis informed by international guidelines and expert interviews (Study 1, N = 3), and assessed key tasks regarding expectancy-value-cost ratings by nautical officers (S2, N = 65) through online surveys. Seafarers rated tasks such as tidal- and weather routing highest for energy efficiency, despite associated costs. We further evaluated psychological need satisfaction at work and preferences for human versus automated control in EERP. Autonomy satisfaction was rated significantly lower than other needs, and seafarers prefered automated information processing, but human control over decision selection. Finally, we conducted interviews using the Critical Decision Method with seafarers (S3, N = 22) after EERP with a Decision Support System on a professional ship-bridge simulator. Interviews revealed the complexity of balancing goals, especially safety, and highlighted practical experience as a crucial factor in route planning and system use. All studies underlined that effective support requires high transparency and controllability of system information and functions. Our research emphasizes the importance of understanding seafarers' perceptions of energy-efficient operations and closely integrating automated support into existing processes.