Use your newly acquired boat to get rid of an enemy unit. You will only have to take out the gunner in order to ensure your safety. Start moving towards the southern AA emplacement (use my map if you don't know where it is). Leave the boat somewhere nearby and get ready to proceed with an assault.
You will have to move on towards this small hill. I guess I won't have to remind you to be extremely careful here. If you've done everything according to the plan, you'll be able to surprise enemy units from the east. Try killing all of them from here. Use the scope in order to maintain a safe distance. You can always go back to your boat if you see someone heading your way.
Start heading west. As a result, you will be closing in on the second main target of this mission. I wouldn't recommend destroying the second AA unit right away. Instead, go even further to the west. Take out a few additional enemy units. You should also be careful, because there's a second patrol boat to the north. Avoid it at all costs.
You will also have to neutralize one of the sniper towers. It can be found to the west. Otherwise you would have some serious problems exploring this area. You'll find a lot of C4 charges here, as well as ammunition for your main weapon.
Now you may finally go back to the second AA unit. Approach it and place a C4 charge on the AA gun. Move back to a safe distance and detonate the charge. Now you will have to wait for a plane to get here. As a result, allied soldiers will start taking over this area. You won't have to assist them in securing this section of the island.
Now you will have to try and get closer to the third AA gun. As a result, you must go south-west. Try staying close to the water. You will have to swim towards a nearby island (screen). Make sure that you've avoided an enemy patrol boat. One of your allies should contact you here. He will inform you about a possibility of fulfilling an additional mission objective. We'll take care of this problem right away, especially since you're very close to your new destination.