I also have a secret passion for fortification names, sometimes useful for fantasy: barbican, bartizan, caponier, counterscarp, courtine, demilune, enceinte, fausse braye, glacis, mamelon, motte, ravelin, redoubt, tenaille
(Yes, I know, fantasy in which
cannon or their analogues work).
Flotsam is the floating remains of a ship or stuff that was on it. Jetsam is floating stuff that was deliberately tossed from a ship (think "jettison").
This is my understanding also. Jetsam was tossed.