Upon accepting this contract, a red skull will appear on the map in the centre of Salt Lagoon, an island to the northwest of our current location. Sail over here and when you arrive, park in the shallow water and swim across to shore.
You’ll notice a green investigate circle in the map. Our target is located within this. Fortunately, it’s only a very small circle and there is only one person on the island, so it shouldn’t be too difficult to locate him.
Once you have found him, run up and assassinate him to fulfil the contract.