I was just in Venice! Personally, I think you are wasting your space in the bag bringing a telephoto lens to Venice. There were not too many opportunities I discovered where I personally wanted anything more than a 24-105. I took a few shots with a 16-35 (it was actually the Tamron 17-35) but no need for reach. The city is very small and has narrow lines of sight unless you're on a boat.
I hope you had a nice time. We were there during the awful flooding. The first day of the flooding it was kind of interesting, but the second day sucked -- everything was closed down, and it was hard to find food. I felt bad for the Venetians, but I think they are used to flooding
Fortunately for us the weather improved the next couple of days were were there.
I actually used the 100-400 quite a bit in Venice. I focused in on small detail areas and from the top the Bell Tower immediately across from St. Mark's Square.
I also used the 11-24 a lot too, as well as the 24-70. I found uses for all of them, and I'm glad I brought all three lenses despite the extra weight.