I have a Celebrity cruise out of Ravenna on 9/15 and was just given this info by my travel agent (VTG): I was told to arrive no later than 10:30am. On Google Maps, it looks like this is the location described: https://goo.gl/maps/7ZFukwHnTFqmiJYx8
The pick-up point is located at the end of the People Mover, which is the elevated tram that takes people to and from the island.
VENICE CITY CENTER 11:00AM PICKUP
ISOLA NOVA DEL TRONCHETTO
VENICE, ITALY
MEET CRUISE LINE REPRESENTATIVE BY THE "PEOPLE MOVER",
STATION ARRIVAL - GROUND FLOOR
Hope that helps!