Flotsam is floating wreckage of a ship or its cargo.
Jetsam is part of a ship, its equipment, or its cargo that is purposely cast overboard or jettisoned to lighten the load in time of distress and is washed ashore.
Lagan (also called ligan) is goods or wreckage that is lying on the bottom of the ocean, sometimes marked by a buoy, which can be reclaimed.
Derelict is cargo that is also on the bottom of the ocean, but which no one has any hope of reclaiming (in other maritime contexts, derelict may also refer to a drifting abandoned ship).