This work presents a framework for realistic power analysis of a fuel cell-powered vessel, with a focus on determining the required power rating and sizing of the hydrogen storage system. Both compressed and liquid hydrogen storage options are evaluated. The analysis accounts for system inefficiencies, auxiliary power consumption, fuel cell degradation, and safety margins to ensure robust plant sizing. A simplified case study demonstrates that the fuel cell system must deliver over 50% more power than the vessel’s operational demand due to parasitic and balance-of-plant losses. Furthermore, to accommodate fuel cell degradation over time, the installed capacity must exceed twice the baseline power requirement. This highlights the critical importance of incorporating real-world constraints into the design of hydrogen-powered marine propulsion systems.