The Grand Canal Boat Tour was magical. I booked for 4 of us and 2 others joined the tour. We had a delightful tour guide who was genuinely warm and very informative. I booked on line and the instructions as to where to meet and submit your voucher was very straightforward. However, I did notice that it would have been cheaper to book at the booking office at St Mark's Square. I am not sure if this would be the same standard as the Viator company so maybe it would be a risk. Despite the slightly extra cost I would book with Viator again. A truely memorable journey. (p.s. it was an evening tour and the weather was glorious and no swell).