Actually, jetsam is thrown overboard to lighten the ship during a storm (or, I suppose, containers that break loose and wash overboard ditto). Flotsam is floating stuff left over from a wreck. It makes a difference in admiralty law.
I was never a big time admiralty lawyer (too far from the ocean) but I did have a couple of admiralty cases awhile ago.
Not as an excuse, but just to say that my speaking out of turn stems from my knowledge/distinction on/of flotsam/jetsam being taken from how my dad explained it to me as a girl. And I'm sure that it was me, not my dad, who misunderstood.
And I have to tell you an LOL. When we kids were little, my dad made a really neat, HUGE floating raft for us, for diving and for our motoring-around pleasure (he built a mount for a three and a half horse motor on the "back.") I'll tell you more about it, including the very funny name my dad dubbed it, via FReepmail. I guess this story really isn't yet an LOL. Sorry, I'll finish by mail. :-)