I agree with Ebon in bringing throwing daggers. Those are a big help. But I may suggest keeping to the many floating boats and poles instead of touching ground. Time your movement so that the drunks don't knock you off. Or if you're up to it, grab and throw them. ;) Take out the platform guards with your throwing daggers if you can. Head for the back of the ship aka the quarterdeck area.
(Go to http://blognomic.com/images/ShipParts.gif
for a ship diagram)
You should see a pole right under the quarterdeck. Jump from it to the ship's side and wait.
Sibrand is a paranoid maniac at this stage. He rants and raves, threatening you from afar (without knowing where you are ;p) and loosing arrows off into nowhere in particular. But he always does it at the quarterdeck where no guards are posted (when I did it anyway).
Keep hanging on the side of the boat until Sibrand looses his volley of arrows and starts heading back down to the main deck. Quickly climb up onto the ship and rush at him from behind with your assassin's blade and take him down!
Escape the guards and return to the Assassin's bureau (as usual) and claim your achievement!
NOTE: this is also the perfect way to score a full health kill for your Disciple of the Creed achievement :)
UPDATE: please double check for me to see if throwing drunks in the drink desynchs you at all. There may be a concern of drunks being civilians, after all. ;> See comments for details. Thanks!