First and foremost no plastic should be thrown overboard under any circumstances. Food that has come in plastic wrappers, rinse the wrappers in salt water store in a plastic bag until port us reached and the plastic can be properly disposed. If the bag that the plastic is being stored in begins to smell pour in a little Clorox or other bleach and seal bag tightly. Plastic water bottles crush and pack for disposal ashore.
In the galley there should be three garbage receptacles, bio degradable, sinkable and plastic.