Flotsam and Jetsam

At the cabin the other weekend we wondered about the difference, if any, between flotsam and jetsam. To be honest, I don't think I've ever heard one word used without the other. I promised I'd look it up. According to Webster's Ninth New Collegiate Dictionary flotsam is "floating wreckage of a ship or its cargo; broadly: floating debris." On the other hand, jetsam is "the part of a ship, its equipment, or cargo that is cast overboard to lighten the load in time of distress and that sinks or is washed ashore." If the debris floats, it is flotsam. If it sinks or washes up on land, it is jetsam.