...so, is flotsam the stuff which floats, and jetsam the stuff which...doesn't?
Well, not exactly.
I was afraid it couldn't be quite that simple...
Look, jetsam is jettisoned - that is, it's thrown off the boat during storms. It's done to lighten the load to give the boat more chance of surviving. Jetsam: jettisoned. See?
Flotsam, on the other hand, is the stuff you find if throwing all the jetsam overboard doesn't helped: it's the floating left-overs from a wreck.
I see. So how are we supposed to spot either of those?
Well, if you're not at sea at the moment, flotsam (or flotsam and jetsam) also means useless or discarded objects (which are everywhere) or sometimes displaced people such as vagrants or refugees.
Horribly, they're not rare, either.
Spot the Frippet: jetsam. And flotsam. Jetsam is short for jettison, and is from the Latin jactātiō, a tossing-about. Flotsam is from the Anglo-French floteson, from floter, to float.