Objective 1: Search Database For Information On Secure Area
Follow the guard to your left, when he turns around the truck continue forward until you reach the building with a security camera on top of the door. Sneak while the camera is rotating on the other side and pick the lock of the door. Sneak behind the single guard and break his neck. Head to the other room and do the same on the other guard. Go to the room with the computer and kill the guard inside.
Objective 2: Turn On Power To Crane
Go out the same way you entered and head to the northern area with lots of cargo containers in it. Check your map, you'll see that the southern section has a gap between two containers where you can get in. Sneak behind the three patrols and head to the space. Move around until you reach the guard house, from here you can kill the guards without them noticing you. Peek then pop out to kill each guards. You might start with the guy inside the guard house. When the three patrols see their dead bodies they will run to the alarm, you can shoot them one by one if your hiding beside the guard house.
Head to objective 2 building, activate the panel on the gate while the camera is facing the other side. Move not directly to the generator for you'll be seen by the sniper on the roof. Walk around far right and along the wall. Get behind the building and climb the ladder up. Sneak behind the guard on the roof and snap his neck. Pick his sniper rifle and jump down to the generator (you'll only loose a little health anyway). Turn on the power and head out while watching the patrolling guards outside. Exit the compound the same way you got in.
Objective 3: Destroy Secure Compound Wall
Get close to the crane by moving along the wall and around the crane. Sneak behind and (as usual) break his neck. Climb up and operate the crane. Before you get down and enter the secured compound I suggest that drop your rifles/Socom here. You won't be needing them yet but they will be useful on the last part of the mission. SO BETTER LEAVE THEM HERE.
Objective 4: Search Offices In Secure Warehouse
Climb down, enter the secured compound and sneak past the patrolling guards. There's a door on the southern side of the warehouse so head there. Crouch your way to the office and operate the computer.
After the cutscene slash the guard on your front and jump (that's right JUMP!) to the water and swim back to the crane (there's a ladder on the side). If you're following this walkthrough then you still have your stuffs on the crane. Pick it up and if you want you can aim at Quest and his men from here. The problem is when you climb down the crane there'll be more enemies below because of the noise you just created. So you can enter the secured compound first then kill Quest and the others.