In the present investigation, the degradation of high-density polyethylene (HDPE) in marine environments, under ultraviolet (UV) radiation and saltwater conditions, was examined. HDPE debris were collected from the coastal areas nearby Korinthos, encompassing a wide range of exposure durations, from relatively intact to several decades of exposure in the marine environment. The debris were examined via several microstructural examinations and additionally tensile specimens were extracted to evaluate the weathering effects on the mechanical properties. The experimental test results revealed a significant decrease in the mechanical properties that ranged according to the estimated exposure of the debris to the environmental exposure. The debris were classified into three different groups, based on the decrease of their mechanical properties, to be used for recycling. This high-end discrimination of the induced damage to the material properties will probably allow for the appropriate recycling of such debris without essential decrease in the mechanical properties, as currently a small portion of the damaged debris are being used during recycling of such materials.