Welcome to this web site about Venice during high tide, "acqua alta", when the lowest areas of the city are flooded by few inches of water. In this site you can find the prediction of the level of the tide and some pics about this phenomenon.

"Acqua alta" is not a dangerous phenomenon, water invades the town very slowly from the canals and the rising tide lasts only for some hours. It occurs in autumn or winter seasons and are most likely to happen in November and December.

High tides cause very limited inconvenience to Venetians and tourists. Some elevated platforms are set along the main streets to allow passage. In any case access to most of the town is guaranteed.
Only when exceptional high tides occur (higher than 120 cm on the mareographic zero) the wellington boots are really needed, but even on these occasions the inconvenience last just as long as it takes for the water to go down again, which usually happens in a few hours.

However my suggestion is to not forget your wellies and visit the town in a quite unusual way. Wearing the rubber boots you have the freedom to go around the city without the need of the elevated platforms and you can took very particular pics of the flooded city.