Q:

How do I kill the merchant in the "Port Authority" mission without being detected?

One of Ezio's assassinations requires me to kill a merchant hiding on a boat without being detected by any of his guards.
He's being guarded by an archer on the dock (no problem), two patrolling guards on the back of the boat (no problem), two stationary guards near the wheel (kind of a problem), and two heavy guards right in front of him (huge problem).
What strategy, tool, or combination do I need to use to off this guy?

A:

Here's how I eventually succeeded at it.

Jump down from the roof, kill the guy on the dock from the water.
Climb up onto the rear of the ship, kill the patrolling guard when he comes by.
Same thing, different guard.
Hop off the ship, climb down, pull this guard, climb back up.
Wait for guard 7 to be walking away, then kill this guard...
...and this one at the same time, with the double assassinate skill.
Here's the tricky part.  Jump back over the side of the boat and wait until this guard is walking away.  Pickup the body of guard 5 and toss him overboard.  Go back over the side.  Wait.  Do it again for guard 6.  This time, allow guard 7 to spot you before you go over the side.  He'll go into suspicious (yellow) mode and walk over to investigate the spot where you were.  There won't be any bodies so he won't go into alert, and you can haul him over the side.
Now that you have free run of the rear of the ship, hop up onto the rail and air assassinate.

A:

Taken from this post:

1) Dive into the water, either from the rooftop directly (move along
  the roof to the right of the mission start-point) or work your way
  down to ground level
2) Swim to the other side of the boat- DO NOT get too close to the
  boat until you are round the other side
3) Climb up the hull to the right-hand side section of rigging just
  until you are able to grab the top railing
4) Shimmy across until you are towards the back of the boat, just
  behind the 2 guards that are over-looking your target
5) Wait until all patrolling guards are away from you (you can try and
  ledge assassinate some, one of my friends said this worked however it
  did not work for me)
6) Climb up and immediately drop onto the deck
7) IMMEDIATELY drop a smoke bomb- you should have time to do this just
  before you are spotted
8) Run past all guards, jump onto the rail above your target, and air
  assassinate him... DONE!

Alternatively, as user bobbyrk puts it:

The way I did it, was first to swim around the front side of the boat,
  away from the dock. Climb up the side of the boat, and when the first
  guard gets to the back left corner of the boat, pull him over the
  edge. 
You don't have enough time to slide over and pull the second rear
  guard overboard, so don't bother trying. The guard on the dock will
  return, you have to be about three meters from the rear of the boat
  for him to not see you when he reaches his stopping point at the end
  of the dock. When he turns and walks away, monkey over to the back of
  the boat, take out the second rear guard, then jump to the dock, and
  use the gun to take out the dock guard. That'll draw the attention of
  the brutes and the guards at the front of the boat if you time it
  right.
Get back in the water, and climb over to the left front side of the
  boat, near the forward stopping point of the patrolling guard on that
  side. When he gets there, pull him overboard. Repeat that process for
  the other patrolling guard. Don't bother with the two guards in the
  front end of the boat.
Circle to the back end of the boat, climb up and walk slowly up behind
  the two guards in front of the wheel. Walk between them so you stab
  both at once, and you'll be free to take the target out.

A:

Everyone made this mission harder than it had to be...
Climb the back left wall of the ship. Off the ledge, drop a smoke bomb. Move into position behind the 2 guards in front of the ledge above the target and drop a 2nd smoke bomb. Climb the ledge, then jump down for the assassination. 
Piece of cake.

