An easy way to enter the restricted area for this settlement undetected is to follow the coast to the north from the docking area and swim around until you are on shore north of the red zone. From here, look for the rowboat icon on the map. Right next to this is a stalking zone with a hole in the fence in the centre of it – easy entry to the max!
The first sniper is in the guard tower right in front of you so climb up and deal with him quickly before looking towards the viewpoint and entering the vegetation here. You’ll want to be quick to knock out the first fellow and get in the grass to avoid being spotted by a second sniper in a tower in the distance (don’t worry we’ll finish him off last as we are already at the next target!).
The captain will be patrolling around the tents here (tag him with eagle vision at this point) and the powder barrel that we need to destroy is also located in this area within one of the tents. You should be able to make out a powder keg on the ground next to a guard between the tents from the stalking zone, so shoot that to take out the barrels. Now take out the captain with a stalking zone takedown when he comes to investigate.
Finally, there are two remaining snipers – one on the roof of the building below the lighthouse and another in the sniper tower I pointed out earlier. Take them out however you please to end the settlement takeover.