Excerpt from William Holdsworth’s A History of English Law (7th ed. 1956):

     “The crown had originally certain rights to property found upon the sea, or stranded upon the shore.  The chief kinds of property to which the crown was thus entitled were, great fish (such as whales or porpoises), deodands, wreck of the sea, flotsam, jetsam, and lagan, ships or goods of the enemy found in English ports or captured by uncommissioned vessels, and goods taken or retaken from pirates . . . . After the rise of the court of Admiralty the Lord High Admiral became entitled to these droits by royal grant . . . . The right to droits carried with it a certain jurisdiction.  Inquisitions were held into these droits at the ports, or the Vice Admirals or droit gatherers reported them to the Admiral.  The large terms of the Admiral’s Patents incited them, or their grantees, to frequent litigation with private persons or other grantees of the crown . . . . The Admiralty droits . . . are now transferred to the consolidated fund.” [3]

flotsam:
(17c)

1. Goods an debris, especially those from a shipwreck, that float on the surface of a body of water. — aka floatage; flotage.

     Excerpt from Edward Bullingbrooke’s The Duty and Authority of Justices of the P{eace and Parish Officers for Ireland (rev. ed. 1788):

      “None of those goods which are called jetsam (from being cast into the sea while the ship is in danger, and after perisheth) or those called flotsam (from floating upon the sea after shipwreck) or those called lagan, or ligan (goods thrown overboard before the shipwreck, which sink to the bottom of the sea) are to be esteemed wreck, so long as they remain upon the sea, and are not cast upon the land by the sea; but if any of them are cast upon the land by the sea, they are wreck.“ [4]