Go to the airport, fly to the dam, and then parachute out. Your plane needs to go through a red corona in the sky before it lets you jump out. After you have landed near the dam, all your weapons will be removed. Pick up the knife near the containers and try to stealth kill the two guards. Get in the building, plant explosives on all the generators. Kill the mechanics and guards if you have to. Despite the warning that the whole warehouse will be notified of your presence only one of the mechanics has a gun. CJ will dive from the roof of the building into the water. Steal the skimmer plane or the sea sparrow and go where you need to.
The corona is very high in the air so once you take off, fly south and get some distance for you to climb up before turning around.
If you land in the red marker after the parachute jump you'll keep all your weapons.
Although stealth is emphasized, you can completely ignore being quiet. You will finish the mission with your leap from the dam and have a star, but no patrol boats will show up to attack you.