Jetsam is wreckage from a ship floating in the water. If you see jetsam, there must've been an accident at sea.
You often see this word in the phrase flotsam and jetsam, and both words mean the same thing: wreckage from a ship floating in the sea. If a ship crashes into an iceberg, there's going to be a lot of jetsam. In a naval battle, blown-up ships will produce jetsam. Any wreckage that sinks is not jetsam: jetsam is rubble that floats. One thing's for sure: no captain wants their boat to end up as jetsam.