On the first Sunday in September, Venice plays host to the Regatta Storica. The Grand Canal is the venue for this event. Dating from 1825, this regatta is real fun to watch. After the opening procession of boats, the races start between caorlines(six oarsmen) and mascarote(2 oarsmen). Venice really comes to life for this event.
If at all possible, plan your visit when there's a boat race going on. I just got lucky seeing the Vogalonga on my trip, but stumbling upon all those colorful rowers in their rowboats, gondolas, & kayaks was one of the most magical moments of my trip! What a spectacular sight! Probably the best picture I got of it is my intro pic, while I was bobbing up and down on a vaporetto, sticking my head out the window. I really can't recommend this enough!
The Vogalonga is a 30 km. international, non-competitive rowing race. It started in 1974 to promote awareness about the bad effects of motorboats in the Venetian Lagoon, and in support of La Serenissima's rowing history and tradition. I read that for the past two years one person (a Venetian!) has actually swam the entire lenth of the racecourse! http://www.vogalonga.com/inglese/index_ing.htm
I believe the Regatta Storica has been around for much longer. It takes place the first Sunday in September, and begins with a parade of historical boats down the Grand Canal, followed by four races.