Upon accepting this contract, a red skull will appear on the map over at Mariguana Island to the east of our current position. A large red, circular restricted area will also now be present in the area. Sail over here and dock.
We now need to enter the restricted area and use Eagle Vision to identify the target. Both times I played this mission I was able to fast travel to the island. From here I could use Eagle Vision to find him at the settlement, just to the west of where we docked on the wooden walkway that runs directly beneath the islands only viewpoint.
Once identified, run over and stab him in the face to complete the contract.