We each bought a 24-hour tourist ticket, at 18,000 lire (9.30 Euros). This allowed us ride the Vaporetti (taxi boats) as much as we wanted in the 24-hour time period. We decided to start out in the morning with a tour of the city on the 41-42 line.
The vaporetti were almost always packed, but by keeping our eyes peeled at stops we were all able to find seats with good views. It turned out to be an excellent way to get good photos and see the city.
Vaporetti are very slow and noisy: they stop about every 500 meters and bang up against the landings, so if you're looking for a quiet, romantic way to see the city, go for a gondola! (They're much more expensive though!)
Vaporetto tickets are available on the canal at the train station (Santa Lucia) and at stores with the "Actv" sign (Actv is the Venice public transportation group).