The Venetians call it “Acqua Alta” or High Water as it literally translates. It comes in 2 forms really.
There is the mild flooding that comes with the tidal lagoon.
This affects a few areas more than others as the water flows up through the drains into the streets. Warning signs for tourists show which areas are most likely to be affected. It’s not a huge problem at this stage, and life goes on.
It looks like this in St Mark’s square:

What you see in the first photo though is the more dramatic where the water comes into the squares from the open water
This can only happen when the tide is high and the water level has been raised significantly either by rain or low air pressure. This leads to the dramatic flooding we see now when the water mark around St Mark’s Square hits 1.5 Metres (over 5 foot). 90% of the city is flooded. Tables have to be erected & raised to enable people to get around. Many homes (especially those on the ground floor) are flooded.

There are some plus points to acqua alta, especially if you’re a wakeboarder:

Youtube link to Wakeboarding in St Marks Square
And some pictures of the Wakeboarding