REWARDS - Respect, $11,000
NOTE - This mission can only be started between 20:00 and 6:00 on the in-game clock.
Get to the blip at the docks and enter the red marker on foot. CJ will automatically jump into the water. Follow the instructions and go through the markers, both on top of the water and underwater. You have to board the big boat by swimming to it, but there are Da Nang patrol boats nearby. Swim underwater to divert the attention away from the patrol boats, so the Da Nangs don't spot you. If they spot you, they will begin shooting at you. Keep diving under and resurfacing to replenish your breath. There are also spotlights on the ship that will alert the guards if they spot you.
Head to the northwest side of the tanker and board it. Board the boat and grab the knife at the top of the stairs. Walk behind the first guard and stealth kill him. Walk behind the second guy when he's closer to the south side of his patrol. Sneak behind the next guy and kill him. Walk up the steps and stealth kill the next guy when his back is to you. At the top of the steps will be another guy, so kill him and then walk into the room to head inside the tanker.
Head downstairs quietly and watch out for the guard in the hallway. Don't worry about killing him, since the red marker is where you need to plant the bug. Press Triangle inside the red marker, then head back outside. Jump over the railing, into the water and swim to the yellow blip. Make sure to swim underwater every now and then to avoid the gunfire from the boats. Once you get to the red marker on the docks, the mission is done.