In Santa Maria Delle Visitazione, in the Assassin's Tomb, you'll come across a large flooded room just before one of the Armor of Altair seals. Pull the lever and peer into the water. Wait a minute or so and you'll see a shadow below. Pull the lever again and peer into the murky depths once more and a tentacle will reach out to grab (and miss) Ezio.
InstructionsCONTROLS: Left Bumper = contextual cameras;
Left Trigger = target lock;
Right Bumper = weapon selector;
Right Trigger = high profile;
Left Thumbstick = move;
Back Button = map;
Start Button = pause;
Y Button = head;
X Button = weapon hand;
B Button = empty hand;
A Button = legs;
D-Pad = quick inventory;
Right Thumstick = look;
ACTIONS;
ON GROUND;
LOW PROFILE;
A Button = steal;
B Button = gentle push;
X Button = attack;
Y Button = eagle vision, talking;
HIGH PROFILE;
A Button = free-run, sprint;
B Button = grab and throw, tackle;
X Button = attack;
Y Button = eagle vision;
HORSE;
LOW PROFILE;
A Button = walk;
B Button = dismount;
X Button = rear up (if weapon is sheathed);
X Button = attack;
Y Button = eagle attack;
HIGH PROFILE;
A Button = gallop;
B Button = dismount;
X Button = attack;
Y Button = eagle vision;
SWIMMING;
LOW PROFILE;
A Button = dive;
X Button = attack (throw knives);
Y Button = eagle vision;
HIGH PROFILE;
A Button = get out of water, fast crawl;
X Button = attack (throw knives);
Y Button = eagle vision;
BOAT;
A Button = step out of rowing stance;
B Button = row
Control infoGamepad/Joystick