Overall the water of Adriatic Sea is so clean and beautiful but I was also surprised to see this stuff. I checked online and found this information which could be the answer to your question, “The Adriatic Sea is seriously prone to marine litter pollution, and it collects about one-third of all the freshwater flowing into the Mediterranean, mainly via the river Po. This study investigated the type and composition of large microplastic debris collected in different sites in the Po Delta area”.