Click here for our most recent post on mosquitos in Venice. Some of the posts in that thread don't address Venice, but the 1st reply (1st after the original post) and the 4th reply do specifically address mosquitos in Venice. And the 4th reply has a further link you can click to get even more info on mosquitos in Venice.
A problem is that, unlike here, the windows in hotels almost never have screens. If you have a room with AC, and if it's the time of the year when Italian law allows hotel guests to use the AC, then you can live (or I should say, sleep) with the windows closed. But if you have to sleep with the windows open, that's when people have had problems.