To begin this mission, head over to where Sam is in Thurston Town. He wants you to join the Privateers, but you must look the part first. In order to do that, you must kill a recruit and steal his uniform. The objective marker is a pretty good ways south, and it just so happens a Radio Tower is on the way, which you should activate.
When you arrive at the camp, Jason will make a note that he should remain hidden until he obtains the uniform. This means that you can't be spotted and you can't kill anyone either. Your first objective is to use the stairs to go down into the cenote, but there's a guard sitting on the chair right next to the stairs. Use a rock to lure him away, then sneak past him.
You can't jump into the cenote, so you will have to walk down the path. You can hear two people talking, both of whom are below you, so just tag them for now to know their locations. When you get to an opening, there's a guy going to the bathroom. Wait until he starts moving and just follow him.
This next part can be kind of tricky, as the guy from before will join with another guy. You will need to throw a rock to the right and time it so you can move to the left of their location when they're not looking. Drop down to the area below and hurry up to the next area. Use the rope to climb to the ledge above, where you will need to distract yet another guard with rocks.
For this guard, lure him into the open area and when you get to the marker, you will hit a checkpoint. Fail on purpose by jumping into the water, which will alert the guards. When you respawn, the one guard will be back inside the cave and out of your way, allowing you to easily sneak past the second without having to distract him with rocks.
Once you make it into the next room, you'll see two guards who will move with their backs to you. Tag them, then go through the foliage on the right and tag the enemies in the next room. What you need to do here is go through the foliage on the left now, throw a rock to distract the guard in the previous room, then quickly go under the ramp and through the opening to the docks. After making it to the docks, drop into the water and swim to Hoyt's boat.
Climb aboard, silently kill the recruit and you'll take his uniform. You'll call Sam and tell him you're ready to go. Go up to the Privateer and he'll scan your ID, and also take you to the side and talk about making money from Vaas' empire now he's gone. Once you're done with him, go up with the other recruits for orientation. After it's done, go in the water and board the other boat, driving it out of the cenote to finish the mission and start the next one.
