No, "tourist season" is not when you get to hunt them. And yes, in winter the city really does flood regularly.
Every year, up to 1.5 million tourists join Venice's 70,000 residents, making the town called La Serenissima ("The Most Serene") anything but calm.
Summer in Venice: Peak season
Frankly, I'm surprised more people don't fall into canals accidentally, like lemmings off cliffsides, from the sheer press of other people pushing them over the slippery fondamente (marble embankments).
37 / 1.5
48 / 1.9
61 / 2.4
78 / 3.1
65 / 2.6
69 / 2.7
52 / 2.0
69 / 2.7
59 / 2.3
77 / 3.0
94 / 3.7
61 / 2.4
Also, with the summer heat comes the humidity—and a city built on the water can summon a mighty amount of humidity, let me tell you.
Also: mosquitoes. Mosquitoes can swarm to plague proportions in July and August. That means you don't want to be opening windows, which means that air-conditioning at your hotel is vital.
(Though that won't stop 90% of hotel maids from flinging open your windows when they clean your room—they do so to air out that funky tourist smell—then leaving them open until evening, when every last blood-sucker invites itself inside. This drives me nuts.)
Venice in winter: Off-season
Then again, travel in the winter off-season presents its own challenges.
Fellow tourists are still pretty thick on the ground (just not as street-clogging choking as in summer), and in winter you often have to contend with the acque alte, the high waters that see the lagoon invade the city streets on a regular basis (peaking in December) and everyone has to slosh around with hefty bags over their lower legs or mince along the temporary boardwalks that are strung along major arteries. (Still, that can be fun, too.) » more
And you know how you can handle cold, but when you combine cold with damp, it get's chilly right down to your bones? Well, just imagine how damp a city built on water gets in winter.
That said: I love winters here—and not just because, for years, guidebook publishing schedules used to have me spending parts of each January or February in Venice. (I know: poor me.)
I appreciate the quiet, the (relative) emptiness, and even the overcast, dreary, and foggy days, which can lend the city a Romantic somberness and mystery it wears incredibly well. I just realize winters in Venice aren't for everyone, so be warned.
Venice in spring and fall: Shoulder seasons
So, as is usually the case, the shoulder season is best, though for Venice I'd pick spring (April and May) over fall (October and early November), if only because, again, Venice is damp by nature (though calling such a waterlogged city "damp" is something of an understatement), and November is the wettest month, so you can really feel those first chilly nights of autumn.