Begin the mission. Take off from LV airport and head SOUTH, towards Los Santos while climbing. When you get over North Rock, or thereabouts, swing the plane around towards the marker, climbing all the time. When you get to the right height, level off and fly into marker. Now you parachute towards the dam. Pull back on the left stick to slow CJ and head for the new marker on the dam. If you splashdown into the water you lose all your weapons, so get it right. After landing on the dam make your way to the generator alley. Two security guards will be near the door. Sneak up on them and slit their throats. Go inside. Once inside make your way to the lower level via the stairs on your right. Watch the guard here, and when he turns his back to you sneak up and slit his throat, then move along and do the same to the next guard on this level. Go back to the stairs you came down and go up them. You'll see a guy working on a machine panel, and a guard nearby. Wait until the guard goes away and kill the first guy with your knife. Go to the back of the first generator and place your charge, watch the guard you saw earlier, and kill him (quietly) at the first opportunity. Sneak down the alley placing charges. Kill by steath the two or three guys left, and exit the alley. Now you're on the top of the dam, and the police have been alerted to you. The cut scene has you jump into the lake a la Harrison Ford in The Fugitive. Now just swim west to the boathouse near the bridge, go to your airstrip and save.