I always suggest making your proposal at San Gorgio Maggiore.
It is an island in front of San Marco Square.
You have to take the pubic boat or a taxi to reach it.
I like this place because it is quiet and their will not be people in the background and the view is amazing.
I will wait for you there and be discreet until the moment you kneel.
This shooting last one hour and finish to san Marco square.
It usually begins 1 hour before sunset.