xunker has already given two candidates for the distinction between flotsam and jetsam. I propose a third. According to The Oxford Companion to Ships and the Sea, flotsam is parts of a wrecked ship or her cargo, or goods accidentally lost overboard, while "jetsam" is restricted to mean goods deliberately thrown overboard to lighten the load. The Companion also notes that in the original legal sense, jetsam is actually the place where the goods are thrown overboard, not the goods themselves. Both flotsam and jetsam must be floating on the surface, as opposed to lagan which must lie on the bottom. "Waveson", if you ever encounter that word, means the same as "jetsam" and is related to "waive", not "wave". Flotsam is also the word for "newly ejected oyster-spawn". Yummy.