It’s not unusual to see the phrase “flotsam and jetsam” used just like it was in 1884: “A mania for buying all sorts of flotsam and jetsam.” That’s always the phrase, in that order. “Jetsam and flotsam” just wouldn’t sound the same. And you hardly ever see “flotsam” without “jetsam,” even though they’re really not the same thing.
“Flotsam” and “jetsam” are related, though, and based on the same sort of event: a shipwreck, or at least an event damaging enough to a ship that some of its cargo ends up in the water. That’s where “flotsam” comes in; it refers to any pieces of cargo from a ship — or, if the worst happened, pieces of the ship itself — that are floating in the water. Some of that stuff eventually washes up on a beach somewhere. When it does, it becomes “jetsam.” And I suppose a good storm could wash some of the jetsam out to sea where it would once again be called “flotsam,” potentially followed again by “jetsam” when it washes ashore somewhere else. But it would be the same stuff the whole time.
There’s another subtle distinction between flotsam and jetsam, but this one is only interesting to maritime lawyers. “Flotsam” was whatever was washed overboard, and “jetsam” was tossed overboard intentionally. “Jettisoned,” that is, which is where the word “jetsam” comes from. The difference is only interesting to lawyers because evidently if you can prove that something was tossed overboard on purpose — even for the excellent reason of saving the ship itself — the eventual ownership of that thing might be differently decided.
Particularly in the days when ships were made of wood, propelled by sail, and were considerably less reliable than they are today, a whole (ahem) raft of laws and, of course, jargon grew up around what to do with “wreck,” which is a legal term for all the stuff left behind when a ship sinks. Another word that had a specific legal meaning in that context is “derelict” — now we think of it as an abandoned vessel, or vehicle, or even a derelict building, but originally it meant the cargo and fragments that didn’t float (flotsam) or wash up on shore (jetsam), but sank and couldn’t be recovered.
If the ship sank in fairly shallow water, of course, you could go back when the weather was good, dive down, and maybe get some of your stuff back. The first task would be to swim down to find it. When you did, you’d probably mark it with a buoy. Once you did that, the stuff at the bottom wouldn’t be “derelict” any more; it would be “lagan.” Even during a bad storm, the sailors might have been able to attach a buoy to their own “jetsam,” as was noted in 1641: “Lagan is such a parcell of goods as the Mariners in a danger of shipwracke cast out..and fasten to them a boigh or corke, that so they may finde them… These goods are called Lagan…”
“Lagan” never entered common usage the way “flotsam and jetsam” and “derelict” did. It’s impossible to know why one word gets used and another gets abandoned, but in this case maybe it’s just because nobody had time to fasten to “lagan” anything that would linguistically float so it could be retrieved sometime later.
