How often have you said that you've seen some flotsam and jetsam when you're out for an afternoon sail? Likely what you're referring to on Lake Michigan is plastic bottles, half-deflated Mylar balloons, or water-soaked, partially-submerged lumber. But what really is flotsam and jetsam?
Flotsam and jetsam are, in fact, two different things, but it's easy to see how they got linked like "ham and eggs." Technically, "flotsam" is defined as goods that float upon the sea after a ship has sunk; "jetsam" refers to goods that have been thrown overboard during an emergency to lighten the ship. So, probably what you're seeing on the lake is neither flotsam nor jetsam, but just someone's carelessness. But when you're out sailing, if you spot any of these objects, it can be a great opportunity to practice your Crew Overboard maneuvers while cleaning up the lake at the same time. And it won't matter what you call it.
But if you spot a trail of what is truly jetsam, you should probably hail the vessel responsible and ask if they need assistance. And if you see what is truly flotsam, look for survivors.