Introduction: The accumulation of plastic debris in marine environments poses an ecological threat, particularly in semi-enclosed basins such as the Mediterranean Sea. Biodegradable plastics have emerged as promising alternatives to conventional polymers, yet evaluating their actual degradation performance under environmental conditions remains methodologically challenging. Protocols, standardized and reproducible, fail to account for the complexity of in situ variables—such as temperature, solar radiation, hydrodynamics, and seasonal fluctuations—which can profoundly affect degradation kinetics. This highlights the urgent need for field-based studies that reflect real-world scenarios. Materials and Methods: We conducted a one-year in situ degradation study at the port of Calpe (Spain), exposing polyethylene terephthalate (PET), polybutylene-succinate-co-adipate (PBSA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and a 70:30 PBSA:PHBV blend, each 200  µm thick. Sampling was performed at 2, 6, 9, and 12 months. We assessed mass loss, biofilm protein concentration, surface morphology, roughness, thermal stability and chemical changes. Results: PHBV showed the highest degradation (41.8% mass loss at 12 months), followed by the 70:30 blend (24.1%) and PBSA (11.2%), while PET remained unchanged. Biofilm’s protein content remained stable for the first 9 months but increased sharply at month 12 across all materials, less markedly in the 70:30 blend. SEM revealed surface damage in all biodegradable polymers, especially PHBV, while PET served as a microbial support. Thermal behavior changes aligned with degradation, whereas FTIR alterations were limited. Surface roughness increased over time in PHBV and the blend, consistent with microbial activity; PBSA showed a more variable pattern, and PET remained unaltered. Conclusion: Our findings confirm the decisive role of polymer composition in driving degradation under natural marine conditions and underscore the need for realistic, long-term field studies to guide the development of truly sustainable plastic alternatives. Acknowledgments: Projects TED2021-130211B-C31 and PID2021-128749OB-C32 were funded by MCIN/AEI /10.13039/501100011033 and by the European Union NextGenerationEU/ PRTR and by MCIN/AEI/10.13039/501100011033 and FEDER “Una manera de hacer Europa”, respectively.