Yes, I shared your initial understanding relating to the deliberate casting overboard, Rod, and now your puzzlement too! This is what the 'quick look' dictionary suggests:
flot·sam (flŏt'səm)
n.
Wreckage or cargo that remains afloat after a ship has sunk.
Floating refuse or debris.
Discarded odds and ends.
Vagrant, usually destitute people.
[Anglo-Norman floteson, from Old French floter, to float, of Germanic origin.]
USAGE NOTE In maritime law, flotsam applies to wreckage or cargo left floating on the sea after a shipwreck. Jetsam applies to cargo or equipment thrown overboard from a ship in distress and either sunk or washed ashore. The common phrase flotsam and jetsam is now used loosely to describe any objects found floating or washed ashore.
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The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reservedAlso, my old Chambers gives this:
jetsam (obs.) jettison: the goods so thrown away and washed up on shore: according to some, goods from a wreck that remain under water
flotsam goods lost by shipwreck and found floating on the sea 
Come in, tsuwm - or should I say Oedipus Rex?
