Grand Canal Cruise in Venice

Walk to the Rialto Vapretto dock and take the No 1 or No 2 boat, direction Piazzale Roma. I am going to suggest that you change at Piazzale Roma to the No 1 boat going the other direction. That is where the No 1 boat originates, so you can be the first people on board and pick your seats. The next stop will Ferrovia where Rick's Audio Tour starts. When you are done, you can walk back to Campo Santa Maria Formosa from San Marco.

Yes, it feels a little counter-intuitive and perhaps even time-wasting, but do as Sam suggests and don't try to do Rick's audioguide backwards or in bits and pieces. That's often fine for a museum tour but I found it much easier to be in sync with the guide at all times for the Grand Canal Cruise.

Some boats (the older ones, I think) have a few seats in the prow, which is where you get the best views. The next best option is the outside rear of the boat. If you are inside, you can only see one side of the canal, and if you are standing in the boarding area (between the pilot and the inside) you will stand for the whole ride and only be able to go from side to side if the boat isn't crowded.

For me the best times are early morning and late evening when there's little traffic on the canal and few passengers on the boats.

Better be very early in the morning as the commuters will be pouring into Venice starting at 7 am and a lot will be arriving at the bus station and taking the No 1 to work. Many more at Ferrovia.
Last time there, I did not see any of the older boats with the front seats in service on the No 1 line. They are not popular with the drivers as tourists keep standing up to take pictures, blocking the drivers view. They spend all their time knocking on the window to get people to sit down. Knuckles get sore after a while.

I think you should have a vaporetto pass, at least for that day, as I believe you will need 2 tickets, one for the ride to Piazzale Roma and one for the ride down the canal. You have to scan your ticket in the machine as you walk onto the vaporetto floating dock. That starts the validity clock for the ticket. A single ticket is good for 75 minutes ride time in one direction, so doubling back would not be allowed. I don't know if the ticket would work in the scanner if you immediately boarded another boat going in the opposite direction. A single ticket is 7.50 EUR, a 24-hour ticket is 20 EUR, 48-hour is 30 EUR, 72-hour is 40 EUR, 7-day ticket is 60 EUR.

They're still running the older boats. I found out about blocking the driver's view when I stood up to get off the boat while she was trying to dock it at a stop... Ooops.

Thank you so much, Sam and everyone!