Polymer electrolyte fuel cells (PEFCs) have attracted attention for marine vessels because of their potential to reduce pollutant emissions on the ocean. Electrochemical impedance spectroscopy (EIS) is suitable for real-time diagnosis for marine PEFCs. In this study, EIS is used to assess the impact of sea salt (NaCI) contamination in the cathode through the fast Fourier transform technique combined with a transmission line model including proton conduction in the ionomer/electrolyte membrane and the charge/mass transfer resistances with associated capacitances as diagnostic parameters. Variations in those resistances from the polarization losses in accelerated degradation tests with NaCI injection are revealed together with the capacitances. Five hours' injection led to ca. 2.5 times larger mass transfer resistance possibly due to lowered hydrophobicity of the gas diffusion layer. Distilled water humidification after detecting the NaCI contamination at an early stage mitigates irreversible catalyst degradation indicated by transmission electron microscope observations.