This is from the Wikipedia page, and is rather fascinating. I particularly like the idea of who owns what, depending on where it is found.
“Traditionally, flotsam and jetsam are words that describe specific kinds of debris in the ocean. Historically the words had specific nautical meanings, with legal consequences, but in modern usage they came to mean any kind of marine debris.
There is a technical difference between the two: jetsam has been voluntarily cast into the sea (jettisoned) by the crew of a ship, usually in order to lighten it in an emergency; while flotsam describes goods that are floating on the water without having been thrown in deliberately, often after a shipwreck.
Generally speaking, jetsam is the property of the finder, while flotsam remains the property of its original owner.
Traditionally spelled flotsom and jetsom, the “o” was replaced with “a” in the early twentieth century, and the former spellings have since been out of common usage.