"A brutal gang of Templar Privateers has taken refuge on a forgotten island near here. Your target is their leader."
if you haven't visited this location before, you will find it blockaded by a Man o War. If you have not dealt with this Man o War and have no desire to, then approach the Island from the West. This will help avoid detection and ensure you can land without incident. Just remember to anchor within 100 meters of the shore. This is a low level Man o War so if the jackdaw has been a good number of upgrades, it will stand a chance.
Again, the best way to start this assassination mission is to jump to the Fast Travel point. Turn around on the Viewpoint and look down to find a target. Hit them with the Berserk Dart. Perform a Leap of Faith into the water after that. From there go along the cliff face to get into a Stalking Zone that puts you in range of your target. Dart him and be done with it.