Make sure you use our camera to scope enemies, then you can see which way they're facing - as well as exactly where they are.
First off; take out the guy in the main look out tower (that you have to go up a wooden path to reach); lookout for snakes near the base of the tower. Once you've got him (silently), kill the alarm then head towards the radio tower. Go down the road which leads to the sea, then follow it round to the left so you'll be behind the guys you want to be killing. You pass a guy hanging on a gallows on your way.
You first come up to two guys at a look out thing - take out one, shoot the alarm (if you didn't get the other one) take out the other guy, then carry on - there should be two more ahead of you down a little hill heading into the camp.
Throw a few stones at the base of the hill to get someone to come over; take him out when he does.
Hide in the hut, then do the same with the remaining guy - you shouldn't have to tempt him all the way back to the hill though, as the first look out guy should now be dead so you should be able to just kill him.
Then head back to roughly where you killed the guys in the second lookout tower, jump in the sea and swim across to the opposite shore; take out the two guys here as stealthily a possible, then head back on yourself and up to the highest point on this side of the island (there should be a hut and some loot here too). From here, get your camera out and scope the area to see where remaining bad guys are, and it should be simple from there..
until you get to the prison.