This side mission begins on the island just west of the Bridge Control outpost. Some of the villagers have gone missing, supposedly going to a sacred pool and never coming back. The NPC wants you to go there and investigate what’s been going on.
The marker is not too far north, and leads you into a cave. Swim down and you’ll see some missing villagers in the caverns below. Once you spot them, go back to the exit and you’ll be attacked by a crocodile. Mash the button to kill it, exit the cave and report back to the NPC to finish the mission (reward is $500).