A more thorough explanation:

Definition: Jetsam refers to goods that have been abandoned at sea and have sunk, remaining underwater. It is also known as jettison. Jetsam is different from flotsam, which refers to goods that float on the surface of the water, and lagan, which refers to goods that have been deliberately thrown overboard with the intention of retrieving them later.

• A cargo ship carrying a load of electronics encounters a storm and some of the containers fall overboard. The containers sink to the bottom of the ocean and become jetsam.
• A fishing boat that has been damaged beyond repair is left to sink. The boat becomes jetsam.

These examples illustrate how goods that are abandoned at sea and sink become jetsam. Jetsam can be a hazard to navigation and can also have environmental impacts if the goods contain hazardous materials.