Use the noun lagan to describe a ship's discarded or lost cargo that has sunk to the sea floor. Sailors will often mark the location of lagan in order to return and recover it.
In the language and law of the sea, there are many terms for things that fall or are thrown overboard, or that remain in the water after a boat capsizes. One of these is lagan, which specifically refers to cargo that can be reclaimed, despite the fact that it's on the bottom of the ocean. Other words are flotsam, jetsam, and derelict, which is something that can't be recovered from the sea floor.