The objective of this study was to evaluate the biodegradation of Poly (hydroxybutyrate) (PHB) and high-density polyethylene (HDPE) in static systems, using as fluid the seawater of the Coastal Region of the State of Pernambuco (Brazil). The physical and chemical modifications of the polymers, as a function of biodegradation, were evaluated by Fourier transform infrared spectroscopy (FTIR), mechanical tensile assay, differential scanning calorimetry (DSC), gravimetric test, and microbiological analysis. Through the FTIR, it was possible to observe in the PHB a decrease of 23.22% in the carbonyl index for the crystalline phase and 32.30% in the amorphous phase after 180 days, which evidences the effect of the biodegradation present. The mechanical properties of PHB were altered with biodegradation, but the thermal properties remained. During the gravimetric tests, there was a reduction in mass and consequently higher degradation rates for PHB, which is corroborated by the microbiological tests of the system. All characterizations demonstrated that the surface of the HDPE is less susceptible to biofilm formation and, consequently, to the enzymatic action of microorganisms. After 180 days of immersion, no significant microbiological degradation was observed in the HDPE, except for some abiotic alterations.