The Hidden Blade Assassin's Creed in Depth

Mysterious Death
Start off by using the passing crowds to hyperblend on the horse. If you are not familiar with superblending and hyperblending, please see these articles first:
http://www.thehiddenblade.com/superblend-exploit
http://www.thehiddenblade.com/hyperblend-exploit

High profile dismount into the water and swim to the ship. Most of the time, my dismount puts me directly in the water, but sometimes I land at the edge of the water and take a single free step before jumping in the water. This single free step does not disrupt the hyperblend. Climb up side of the ship and wait for the target to pass by. Hop down, poison the target, and quickly climb up a rail to superblend before the blending grace period wears off. I use a low rail for the superblend so that there is less delay in climbing up.

Spring-loaded Air Assassination
Again, start off by hyperblending (or superblending) and swimming to the ship. Climb up the rope ladder and lock onto the guard patrolling the starboard side of the main deck. When you reach the platform, shoot the guard in the location as shown. This will attract the attention of the target, and positions him properly for a spring-loaded air assassination from the tip of the mast.

There is no ship.
As you approach the ship, the game will spawn the assassination target on the deck of the ship. Ordinarily, if you leave the area, the game will despawn the target. And if you go far enough, the game will stop modeling the ship as well.

The goal here is to force the game to stop modeling the ship while the target is still on it, thereby causing the target to fall into the water and drown. You can accomplish this by keeping the camera turned toward the target as you leave - The developers don't want you to see the target despawn, so they enabled the despawning only when you look away.

With the camera facing the "wrong" way, you won't be able to really see where you are going. I use the mini-map and landmarks to help guide Ezio's movement. In this video, I swim away to the south, back to where I started near the lighthouse. With the ship out of view and at a distance, the game finally stops modeling it, and the mission suddenly ends with the target drowning.