The busy season for tourists is long - extending from Easter until early October.  July and August are the warmest months with average maximum daily temperatures of  27 Celsius, and the tourists swarm into Piazza San Marco.   Average daily minimum temperatures are 19 Celsius in July and 19 Celsius in August.  Humidity can be high and the combination of high temperatures and high humidity can be oppressive, particularly in very crowded strets.

 April, May, June and September and October are all much pleasanter months for weather with average daily maximum temperatures ranging from 17 Celsius in April to 24 Celsius in September and 25 Celsius in June.

Weather in September is nice; average daily highs around 75F/24C, and evenings around 65F/18C.

High water (alta acqua) is possible at any time of the year but is most likely in the winter months.  During this period the Comune de Venezia (city council) has developed a transportation plan to move about the city while staying dry.  The city provides raised walkways on major pedestrian walkways which are subject to flooding.  These walkways make it possible to walk without getting wet, however it the walkways are narrow and small crowds form at the ends of the walkways.

Winters are cool and rainy.

The best time to go depends on your interests.  There are important events throughout the year from Carnevale in the 10 days ending on Shrove Tuesday, the Festa di San Marco on 25 April. the Voga Longa (a major rowing marathon) on the Sunday after Ascension, the Biennal in odd years through June to November, the Festa del Redentore the 3rd weekend in July, the Regatta Storica on the 1st Sunday in September, the Film festival on 1 days in late August, the Venice marathon on the last Sunday in October etc. etc.

Here is a link to the  six day weather forecast information for the province of  Venice, Veneto, Italy.