The increasing demand for improving fuel efficiency of marine transportation has presented opportunities for the development of the power management system (PMS). Different from the terrestrial power system, a shipboard power system contains a large proportion of propulsion loads, which has the characteristics of high dynamics, periodicity, uncertainty, and high dependence on the marine environment. The propulsion load fluctuation induced by sea waves, in-and-out-of water effects, and changeable consumers requirements, may lead to a low power quality and fuel efficiency and it brings challenges in the development of the marine PMS. In addition, the fluctuated load profiles could also be volatile and unpredictable, which makes long-term load forecasting unrealizable and real-time load forecasting essential. To address these issues, a real-time two-layer PMS is proposed for hybrid-powered ship in this article, that can maintain a high fuel efficiency and a healthy state-of-charge (SOC) level over the voyage even in extreme sea conditions. To adapt well to the fluctuated and changeable load condition, a novel multistep load forecasting system is integrated to make accurate load forecasting in a very very short-term time scale (centisecond). Multiple cases studies are conducted under different cases of voyage time, sailing speed, wave conditions, and submergence ratios. The results show that the proposed PMS can significantly reduce the power tracking delays, improve the fuel efficiency, and maintain a healthy SOC level.