Venice is normally visited all year around, even though Spring and Autumn are the most popular seasons for visiting, even though those are also the seasons most subkect to 'acqua alta', so when part of Venice is flooded by the lagoon. Not all Venice is flooded, only some parts, of which Piazza San Marco for sure. As a reference for acqua alta, this website offers live info (in Italian only) http://www.comune.venezia.it/flex/cm/... in case of acqua alta many hotels offer free boots to borrow, if you are worried about it, you can just check with the hotel.

Summer is hot and damp in Venice and it's essential to drink plenty of water. The narrow streets and alleys offer some shade and fresh air, but it's not enough to avoid dehydratation. Water is sold by street vendors, along with fresh fruit, in the most touristic areas (Ponte di Rialto, Piazza San Marco, and all the streets nearby), don't underestimate your need for water.

Summer in Venice is also season for mosquitoes, which put together with hot and muggy weather can be hard to stand on vacation. Beware, they can really be a lot, avoid keeping the window open at night and the light switched on, they can come in the hundreds!  If possible it's recommended to book a hotel with air conditioned, many hotels, most of them actually, don't have mosquito nets on the windows, then it's essential to keep the windows shut at night if you aren't particularly attracted by mosquitoes.

More info about the city of Venice are available on Venice Travel Guide