"The right hand man of a powerful slave traders is believed to be headed toward the Matanazas plantation for some dodgy business. Find the target and kill him."
Be sure that you have upgraded your mortars some (minimally the Advanced Set) and have a full stock of ammunition for them. Head to the Jackdaw and sail out to the waypoint marker. Use the Spyglass to identify your target, a schooner. It is likely sailing close to the coast of the nearby island. Aim just ahead of it and loose a mortar barrage. Depending on the strength of the mortars, one or two shots is all it would take to destroy the ship or at least disable it. It is up to you if you want to board it or sink it to complete the contract.