In recent years there is some flooding in Venice about 100 times a year. Some flooding on a hundred days works out to about 25% of the days. Flooding is more frequent in November and March.
Flooding there occurs when winds blowing from the south (Egypt) combine with high barometric pressure over the southern Adriatic Sea to push the sea water toward the sea's northern end (where Venice is).
Even if it does flood, it doesn't need to ruin your entire Venice trip. But if it does happen when you're there, it will become a factor that both locals and visitors have to workaround and adapt to. The flooding causes the crowds in certain areas to be worse, because people cluster on benches put down for the flood or in other places where the water isn't so deep.
And the market price of Wellington style rubber boots will go through the ceiling!