Right now (3PM Central European Time), Venice is being hit by the third biggest flood in over a century - in fact I think it is the third biggest flood ever recorded. The water is predicted to surge to 1.60 meters above average sea level, which means that most of the ground in the island will be under 50 cm of water, with some parts of the town under up to 80 cm. 
The strongest high tide in history is the one of November 4th, 1966, with 1.94 meters above sea level. And the second one I recall happened in 1980, with 1.68 meters. In both cases the damage was very large. In the recent past Venice has withstood some improvements, with new pavements in many of the most used walkways, but these sea levels mean that if you want to walk around you need proper fishing gear.

You can see the view of the San Marco basin from a hotel facing the lagoon in this live webcam, or many others you can google online (search for "webcam Venice" or similar stuff).

The graph below shows the forecast for the next three days, taken just now.

Update: in a couple more hours (23.30 local time) the water will come back up and get to 1.50 meters above sea level. At this link you can see a video of San Marco square at the peak of today's high tide.