"A Templar associate is said to be visiting Havana to conclude a trade deal of immense importance to their Order. Find him and put a spot to his schemes."
Head over to the East of the pigeon coop to get to the Restricted Zone for this contract. Jump into the ocean from South of the Restricted Zone and swim up to the area. Swim to the Northern edge of the pier to get into position to get at your target. Just move in using Stealth Swim and climb up the side of the boat. Hang on the ledge and wait for him to come into range for a ledge assassination.