In the Santa Della Vistazoine in Venice there is a certain room that is filled with water.

If you go in there, there is a lever. Pull it, crouch down at the water edge and look down for a minute or two.

You will see a weird shape come across the water.

Pull the lever again and a tentacle will shoot out of the water and try to grab (but misses) Ezio.

http://www.youtube.com/watch?v=z_s555kC1cg