Key points of Map of Crooked Island - Assassin Contracts - Assassin's Creed IV: Black Flag
19. The Poachers
Your target is staying Eastwards of here, on the Mariguana isle. Fast-traveling will make the Jackdaw appear right next to the target. In this case, jump down from the stern into the water, swim underwater to pass by the target, avoid the guards, approach him and kill him. Otherwise, you will have to walk around the guards in a standard way. The target is under a roof so, the regular air attack is out of question.
Another method, a bit tricky, is to let both your target and the gunner next to him, see you, after which you dive quickly. With a bit of luck, both of them will start to approach you, decide that they must have been seeing things and start returning. This will expose their backs If you have been lucky, the target will be, somewhat, lagging behind the guard so, you simply need to run up to him and stab him in the back.
20. The Deserter
No reward for staying out of combat. The target is staying at the Salt Lagoon Northwards. Sail up, go ashore and approach the designated zone. The target is all on his own and he will not spot you at first so, try sneaking up to reach the shooting distance and shoot him. There are a lot of thickets on the island, which will help you take cover. Otherwise, you are up for a run across the island.