If you take the Carnival transferbus your luggage will be on the bus with you and you just drop them off at the porters at the port who will take care of them you see them back infront off your cabin you even can leave your carry on make sure that you have all the papers/passport/ect. That you need to board in you handbag.
When you are at the port of Venice there are taxiboats called vaporetto this is like a bus service that can bring you to Piazza San Marco it will cost you a few Euro's for a round trip.
About the boarding time that really depents one time you can board early another time you can't but as long as you get your luggage taking care of your free to explore Venice.
What date are you leaving? We are going to book for the Sept.8th itinerary
If you have any other question just let me know maybe i can be of help.