Marco Barbarico Preassassination
http://www.youtube.com/watch?v=853X806LhiQ
Here is a description of the video.
Preassassinations: Marco appears on the boat right before the scene starts. I try a few different ways of killing him, but he always comes back to life during the scene. When I use the hidden blade there isn't any evidence he's been killed. When I use a sword I am locked on to him during the scene. When I use the gun guards come to where his dead body was. 
Prepoisoning: Using poison is the only effective assassination on Marco before the scene. However he isn't affected by it until after the scene. 
Bonus Feature: As you can see it is possible to have two Dante Moro's at once. 
After you steal the golden mask from him fight him then lead him to the carnival. When you are there escape him to become anonymous and hire courtesans to lure him in the right place. I was lucky enough to have both Dante Moro's right next to each other during part of the scene.
Faster Blitzes?: It might be possible to do a faster blitz on Marco by starting to perform an air assassination as the scene is starting to end.
Loved the prepoisen part, made me laugh. Not sure if you read my comment but try make a vid where you become incognito, I have done it before took a while.
Great video, I love the reactions of the guards on the boats, "just stand and smile" 
Thanks to SBIzokronus for the fantastic sig!
grt job again aurel!
I tried the prepoisoning trick on Dante in Birds of a Feather (i.e. the first scene of Nine Lives where Dante and Marco get bloodstained shirts). It worked... sort of.
Ordinarily if you poison Dante during that mission it's instant desynchronization. With prepoison, Dante behaves normally during the cutscene and then flails around afterward. He doesn't draw his weapon. The other Templars wait around and watch him die. Then Ezio desynchronizes - "Target Died".
It's interesting but disappointing. I wanted to see the Templars continue the mission without Dante.
What if you tried throwing one of the templars in water before the last scene?
It's possible to do a mysterious death on Marco with prepoisoning. But when I did it I preassassinated two guards in case they would have detected me. However there is a way to avoid this: Hire the courtesans by the boat. Wait until a group of guards pass by and distract them with the courtesans. Some of the guards on the boat will get off and also be distracted. And the poisoning needs precise timing. You want to poison Marco as soon as he appears (when the timer is at zero), or it will be too late. You will know if you are successful because you will hear the distinctive sound of poisoning.
What if you tried throwing one of the templars in water before the last scene?
Are you talking to me? Throwing any Templar in the water during Birds of a Feather is an instant desync.
You can throw a smoke bomb and try out various things like punching and grab/throw. A Templar death/knockout = Ezio desync. The only exception I've found is the one in the video immediately before the cutscene. Theoretically you could do something before each cutscene if your timing was perfect. One time I managed to air assassinate Dante going into the second cutscene. You couldn't see it (screen fades to white) but you could hear the assassination and the crowd's reaction.
Yeah, perfect timing. You can also try throwing someone into them before a scene.
You can also try throwing someone into them before a scene.
For what purpose?
You can get away with quite a bit of high profile behavior after throwing a smoke bomb. When the smoke begins to clear you can throw another and continue. I tried repeatedly grabbing/throwing Dante in order to separate him from the Templars. My goal was waypoint separation (Dante gets lost and can't rejoin the group). I never succeeded.