Objective To learn the failure behavior of organic coatings under alternating hydrostatic pressure in deep sea environment and provide guidance for lifetime prediction of the coatings. Methods The diffusion behaviors of water under atmospheric pressure and deep sea alternating hydrostatic pressure were investigated by traditional gravimetric method and electrochemical (EIS). The corresponding diffusion coefficients in different environments were calculated and the effect of alternating hydrostatic pressure on the kinetics of water adsorption was also analyzed. Results Alternating hydrostatic pressure accelerated the diffusion process of water through the coatings, and the diffusion mechanism was changed from ideal Fick diffusion under atmospheric pressure to non-Fick diffusion under alternating pressure. The saturation time was prolonged and the saturated percent of water adsorption was enlarged by alternating pressure. Microscopic morphology observation showed that a large number of cracks appeared along the pigment/ binder interface inside the coatings, which was the main reason for the changes of water diffusion mechanism in the coatings. Conclusion Alternating hydrostatic pressure significantly accelerated water diffusion through the coatings by the pushing effect on water and deterioration effect on the coating structures.