Beached Trireme Camp (Location)
You raid this outpost in force during the Rebel Strike sidequest in Faiyum. Tackling it then does make killing the captain easy- he'll go down in the melee with everyone else (you'll also pass through during the Main Quest for the Crocodile, though without the military backup).
The treasure is in the command tent on the more southwesterly trireme.
Looting the red chest therein completes the location.