According to the Koppen Climate Classification, Venice is in a region classified as a Humid Subtropical Climate. What this means is Venice will have cool winters and very warm summers. The yearly precipitation is spread evenly throughout the year.
The above picture shows the average rainfall and average daily
maximum and minimum temperatures of Venice, Italy. As you can
see, there is a small range in temperatures year round.
Some reasons for these mild flux in temperatures could be due to Venice being at sea level and its continentality. The continentality of Venice plays a key role in its small flux in temperatures from winter to summer because areas closer to water typically have more cloud development keeping the area hotter and more mild in temperature year round.
Above is showing sailboats in the lagoon in Venice with a dark cloud coverage.
Clouds are a way to keep UV rays trapped and therefore making a warmer
temperature.