Post-consumer flexible packaging abandoned in seas and rivers represent an important environmental problem: their recovery is still a challenge and the management of the recovered material is equally difficult since, due to their heterogenicity and to their long-time permanence in seas or rivers, these plastics present various issues related to high incompatibility, hygroscopicity and presence of bad smells. All this translates into poor processability and mechanical performance and low organoleptic quality. The aim of this work is to characterize the seas and rivers plastic waste (SRPW) composition and to perform a preliminary investigation on the effectiveness of two different mineral fillers as reinforcing and compatibilizing agents for SRPW, in view of possible second-life applications of this waste. In particular, an organo-modified montmorillonite (Dellite 67G) and a natural sodium zeolite mineral (Zeolite 4A), loaded at 5 wt%, were considered for improving the processability and properties of SRPW blown films. The combined use of a polymeric compatibilizer (polyolefin block copolymer grafted with 0.4 wt% maleic anhydride was also considered, in order both to improve the filler dispersion within the polymer and to compatibilize the main constituents of SRPW. The results demonstrated that the addition of both fillers to SRPW leaves essentially unchanged the thermal stability, but gives interesting improvements mainly in terms of stiffness. The effects are more significant when the nanometric Dellite 67G is added, than for the micrometric Zeolite 4A, especially in compatibilized systems.