*SPOILER* Finally found it. It's in a crater, in the water, in the north end (north = top of the map) of the swamp, almost due north of where the PDA marker is. Just line yourself up with the PDA marker and walk north, you'll walk right over it, after walking through an anomaly of course.
Strangely, it wasn't in a rucksack, just floating in the water.
jambipk
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