Flotsam is the floating wreckage of a ship. You'll often hear it used with the word jetsam, which refers to floating objects that have been thrown from a ship, usually to lighten it before it sinks.
You can keep flotsam and jetsam straight by remembering that flotsam looks like the word, float, and jetsam looks like jettison which means to cast off. Usually it doesn't matter though as flotsam and jetsam are used together as an expression, mostly figuratively to mean to mean things or people who are unwanted or discarded. What about the things that are cast from a ship but sink to the ocean floor instead of floating? That's called lagan.