From the monthly average data, here for each month of the year the weather which you can expect Venice. To be prepared with jackets, umbrellas, boots or... sunglasses.
I-Météo, The weather in Venice: In Italian but very detailed.
Weather in Venice Italy forecast CNN, presented rather well.
The Weather Channel They also give the weather in Venice, detailed enough, with a 10 day forecast (if they believe in it!)
WeatherUnderground.com Also detailed weather in Venice Italy information but with a 5 day forecast.
In Italian on 3Bmeteo.com Scroll down the page to get a very detailed day forecast of the weather in Venice Italy.
In Italian on Arpa Veneto. Can be a little more accurate than the American sites and in any case more optimistic, on average always a few degrees warmer! They even have the state of ozone layer above Venice as a bonus.