I really hope you can figure out a way to see Venice. Sometimes things get so hectic there you don't get to see the city before the ship leaves.
I certainly agree with Paul!
When we embarked in Venice, we were lucky enough to have an overnight there, so we got a nice time to visit the city.
Hopefully, your ship will offer a shuttle to St Mark's Square that will run the afternoon and evening of embarkation. AT LEAST you will be able to have a gondola ride! I always hate to miss meals on the ship, but you might consider having dinner at one of the many places on the square too.
I actually know where some of the better bead shops are in Venice. You can find them in a lot of places because Murano glass is everywhere, and that is where some of the most beautiful beads come from.
You see all kinds of beautiful necklaces and bracelets there - all glass beads. The entire city is virtually a big beautiful shopping mall (but not with the Gap or Kohls, all private stores).
I don't know about dining there unless you have a recommendation, but I would spring for a caffe latte at the Cafe Florio in St Marks Square. (Probably costs about $15, buts its supposed to be the oldest sidewalk cafe in the world - Joe Farcus has dedicated many Carnival rooms to it).
So happy you are going to Venice! We're booked next October for a TA on the Nieuw Amsterdam out of Venice and am looking for information. We plan to arrive a few days early to enjoy the city. Anxiously awaiting your report. I've already started my research but firsthand experience is always best.