In some of our previous experiences, Regent has made available "Regent sponsored" excursions during these port days for the folks "in transit" on B2B cruises. They just don't allow you to book anything in advance. They will usually let you know a couple of days ahead of time what might be offered. As I'm sure you know, their first priority is to get the those disembarking passengers "out of there"! So, usually the tours leave a little later in the morning. Not sure if they will continue to do that or not.

We were in Venice a couple of days on our own before getting on the ship, and we took the Vaporetta (public transportation system) to get to a few spots since I have a little difficulty walking. I think it was only about 20 euros for a day long pass. We also hired a water taxi tour to take us to Murano and Burano - it wasn't cheap - I think about 300 euros - but we split the cost 4 ways. Walking in Venice is pretty easy as long as you have a good map! We got lost several times!