Boarding "Oosterdam" in Venice

This saturday we will arrive in Venice for a cruise on the "Oosterdam".

When we are on board, lets say around 12, is it possible to check out Venice before the ship sails?

Is it easy and quick to go to the main sites?

You would not have a lot of time, depending on your sailaway time....

If you primarily want to visit St. Mark's Basilica and Piazza, and perhaps the Doge's Palace, I'd recommend taking the Alilaguna boat service directly from the port to St. Marks. I think it would be faster than the other option, which involves walking through the port to the People Mover, riding the People Mover to Piazzale Roma, then taking a vaporetto (public water bus) to St. Mark's, with many stops enroute.

If you want more specifics, I'd suggest posting on the ports of call boards for how to do it. Or do some reading online at good sites like this one:

https://europeforvisitors.com/venice/articles/vi-transport.htm

Easy? YES Quick? NO

If your ship leaves Venice at 5 pm, muster drill will likely be around 4 pm or so. You would have to be back before then. So you are basically looking at 3 or, at most, 4 hours to get to where you want to go, visit a bit and come back. You have to ask yourself if it is worth the trouble. You will get a good view of many of Venice's sights on the sail away from Venice, including St. Mark's Basilica and Piazza, and the Doge's Palace.

I just saw in this week's Time magazine that about 55,000 tourists visit Venice every day. Nothing worth seeing is "quick".

We were in Venice last October, for 5 days preceding our Med/TA cruise on the Westerdam. We booked a small hotel for the 5 nights, thinking that mid-October would be less crowded and cheaper hotels. Were we fooled !! On the Saturday departure of the Westerdam, there were 4 other monster-size ships originating their cruises there. Because they were originating and not just a port call, some 18,000 tourists all looking for a hotel room for at least one night before the cruise (smart move) so prices were crazy (300 Euros per night) and St. Mark's Square was wall-to-wall people. We could hardly move. If there's any way the OP could find a way to arrive in Venice a day or 2 earlier, they would have a much better experience seeing that beautiful city.