Venice has been keeping flood records for 150 years, though it is generally understood that the city has been regularly flooding since the very beginning. It usually happens between the fall and the spring and the earliest record is believed to be from the 6th century.
This past week, Venice saw its acqua alta (or high water) peak at 187cm (6'2") above its normal level. This is the second highest number on record and is just below its 1966 peak. At these numbers, about 80% of the historic center is underwater. Here is a chart from the WSJ explaining that: