Jetsam
Noun: Unwanted material or goods that have been thrown overboard from a vessel. Often to lighten the ship during a storm.
Example: “To try and save the ship they heaved the cannon overboard creating a large jetsam field.”
So to answer last weeks riddle: flotsam is accidental and jetsam is deliberate.
Photo is of a Japanese cannon from WWII rusting away in Palau. Taken in 2015.