A Mariners term for, you know, the stuff that floats in the water (in this case, the oceans of the world). Flotsam is the name for natual stuff like kelp, dead fish, etc. Jetsam is reserved for man-made stuff; stuff that has been jettisoned, hence the name. Stuff like crates, bottles, corpses and anything else that would be lost overboard.
Our good friend Webster has a few writeups that also state that both are a name for stuff tossed overboard -- 'Flotsam' is for the stuff that floats on the surface, and 'Jetsam' is for the stuff that doesn't. Now, I'm not saying he's a liar, but I got my definition directly from the horses mouth; from my aunt who was a lighthouse keeper in Nova Scotia for 35 years. Either one could be right, I suppose.