The older boats have about 10 seats on each side of the boat in the bow, in front of the pilot house where the capitano drives the boat. As such, he is constantly rapping on the the window to get eager photogs to sit down so he can see ahead and not run over any hapless gondoliers and their paying clients.
The newer boats, built under the consultation with said callused knuckled capitani, have no such seats or passenger access in front of the pilot house. These are mostly on the No 1 line.
I think it is at different docks at Piazzale Roma, but they are clearly marked and there is also an electric sign board showing which boats departing in which directions from which docks and their departure times.