So, what happens to the stuff after you flush the toilet? The waste goes through the plumbing into a Type I, II, or III Marine Sanitation Device (MSD) where it gets chopped up by a macerator, and then is subjected to various chemical treatments to make the waste product smaller, and more environmentally friendly.
If you’re at least three miles offshore, you can pump the treated sewage overboard back into the sea. When you’re less than three miles offshore, only effluent that’s been processed by a Type I or Type II MSD can be discharged into the ocean.
If your boat has a Type III MSD (a holding tank), you’ve got to wait until you find a suitable pump-out station before you can off-load your waste products.
When you’re stocking your boat with supplies before your cruise, make sure to bring along some plastic trash bags for refuse, and a handful of plastic grocery bags from the supermarket. The grocery bags are great for collecting recyclables (cans, bottles, etc) to put in the recycling bin when you get back to shore.
Don’t throw any kind of trash in the water, please. It’s nasty, it’s ugly, and it’s illegal.