In Venice there’s a canal where other cities would have a road, and there’s a boat where in other cities there would be a wheeled vehicle. Yet, Venice does not have less traffic than any other modern city of sixty thousand inhabitants.
This makes for a lot of boats.
There are as many different kinds of boats in Venice as there are different kinds of vehicles on land. There are the small private boats, taxis, ambulances, barges for trasportation of goods, busses (vaporetti), car ferries and the occasional cruise ships. Add, on top of all this, the gondolas and sandoli used for the tourists, and every once in a while, a group of kayaks.
The traffic is worst in the Canale Giudecca, Bacino San Marco, Canal Grande, Canale Cannaregio, Canale Novo/Rio Foscari, and the outside canal along the northern side of the city. Those are the places where most motorised traffic is allowed.
In the inner canals of the city motorised traffic is banned, except for specially authorised boats, which include taxis, residents, transportation of goods, garbage collection and police and ambulances. This means that there are very few places where you don’t have to be attentive of motorised traffic.