If your flight is at 11:30 then you should be at the airport around 9ish..meaning to leave the ship at 8 something ( at the Venice port you will have to retrieve your bags from inside the luggage area..that can also be a mess with everyone leaving and the limo/private transfer cars are usually parked right outside that area, so your guy can assist you.
I don't know who you booked with but highly recommend
Http://www.venice-limousine.com
for transfers in Venice.
