I am also not sure what I can do personally on a cruise that is any different than what i do every day. They have different recepticles onboard for paper and plastic - I always choose the right one. I don't throw my empty plastic water bottles overboard.
I really don't get the question, it kind of sounds like a plant from some environmentalist trying to prove people who cruise don't care about the environment.
Its a dumb question - the environmental practices onboard are managed by the ship's staff, not the passengers. They do reduce all waste to ash and dispose of it properly when they arrive in port. I suggest you read up on shipboard environmental practices before you post such a silly question.