I like to think that I take pretty good care of the marine environment when I'm sailing. But as I'm never out for more than a few hours in my Laser it's pretty easy to follow the rule of taking back to shore any waste materials from my snacks or drinks. (Ummm - except of course the occasional need to water the fishes).
But ocean going ships obviously have a whole different set of issues with which to deal. Check out his article on Garbology 101 written by Mary Ann, aboard the Esperanza, the largest vessel in the Greenpeace fleet. I guess being Greenpeace they have to make sure they are ultra politically correct when it comes to dumping stuff over the side, but if you are as ignorant of such matters as myself you might be surprised as what materials they do toss.