the guy on the docks

I found it hard but in the end I found it easier to use the crowd to get near to his boat and get as close to him as you can, throw a few knives at him and run like hell!!!

Here you go try this it worked for me the very first (and I have beaten the game already of course): go to the second dock the one to the left of the one with the guards walk slow jump boat to boat and piler to piler going right stop for nothing!! As you get to bosses boat take a right use your hiden blade kill jump off the front of that boat jump all the way to land then run till you are on "yellow" hide, get on "blue" then continue on.

Go to the back of the boat, and hand there. your target patrols the boat, and he comes up to the back of the boat.
When he turns round and starts to go back round, climb up and use the hidden blade to take him out.

I don't undestand your question.

Get near the ship. Jump to the ship and wait don't climb. Sibrand comes and fires some arrows now climb and use your hidden blade. Use the tower to get out of the ship.

Here's another thing: If you get caught, give chase. You can run faster than him while sprinting, and do free running much better than he can. By holding B, you can also tackle him to set him up for your hidden blade finisher.

Jump thruoh all the little ships so you don't have to worry about the guards climb the back of the ship and make sure you have your hidden blade run up and assainate him