"This Templar associate hopes to regulate and control the rum trade in the West Indies using force and violence. His next rendezvous will take place on Arroyos. Find and kill him."
As soon as you accept this contract, start running immediately for the Harbor Master. Just as you get there you will see a Captain attack and kill the Harbor Master. If you take the flatest route there then you have a chance to identify him then hit him with a Berserk Dart.
If you are too slow then a naval battle is needed hunt him down and finish him off.