This paper presents a novel approach to the production of river buoys, focusing on the use of new composite materials with high recyclability at the end of the buoy’s life cycle. The composites were produced from post-production thermoplastic waste - polyethylene and polyamide 6 - reinforced with glass fibre, and modified using block copolymer polyethylene maleic-grafted-anhydride (PE-g-MAH) to improve the heterogeneity of the waste structure. The composites were prepared by melt compounding with a corotating twin-screw extruder. The melting behaviour, rheology, degree of crystallinity, and moisture sorption characteristics of the rPE/rPA6-30GF/CoM compositions were investigated. Rheological behaviour was analysed using an oscillatory rheometer. Given the operating conditions of the new composites, key functional properties such as specific density and water absorption of the rPE/rPA6-30GF composites were also assessed. The structure of the composites was further examined using scanning electron microscopy (SEM). The results showed that modifying the rPE/rPA6-30GF composite with varying amounts of compatibilizer led to an increase in specific density and a decrease in water absorption, compared to unmodified polyethylene-polyamide 6 composites containing 30% glass fibre. These improvements were attributed to increased homogeneity of the composite structure as a result of the PE-g-MAH modification. The findings indicate that polyethylene–polyamide-based waste polymer composites can be effectively used as potential materials for constructing navigation buoys.