Jenny, I've read your topic several times and keep coming back to the same question: How large is your "medium" bag? Is it carry-on size (21")? Although the RS tours don't really require that size, it's helpful. You will need to carry your bags up stairs and down over canal bridges, from bus to/from hotel to bus, up and down hotel stairs, over cobblestones. Read about packing light and watch RS videos and You Tube hints. Aside from the tour, if you're taking trains, you will need to manage getting on and off with your luggage.
Just lift your bags down the one step on to the vaporetto (after scanning your ticket) and find a place to stand or sit. Since there is another step down to the seating area that's usually crowded, I find if it's crowded just stand with you bags beside you. it's easier to maneuver to get off at your stop. Just like a bus or train you want to be at the exit ready to step (up) and off. Don't expect anyone to help you. Last year, I took the Alilaguna from San Marco to the airport. A nice young man from Argentina took pity on my overpacking and help me carry my luggage on and off the bus. I vowed never to buy so many heavy souvenirs again and to pack lighter to start with.
As others said, don't think of the vaporetto as a boat, but as a bus. If you get a chance, take it through the Grand Canal at night. Venice has a whole different look then, enchanting with the lights reflecting on the canal. I love Venice, have visited twice and hope to visit again next year.
Buon Viaggio!