Found an interesting post on another blog with a map and a complete list of the locations of Venice's 70 or so working fountains with drinking water. Might be helpful info especially if you are traveling to Venice when it's hot. It's also interesting to look at the map - only four of them in the sestiere of San Marco? Seems strange.
People often ask me a couple of questions about Venice - do the canals stink and can you drink the water? The answers are no and yes (the water tastes great, I think).
Here's a contrast between getting water in Venice in the past (the vera da pozzo) and today (the fountain)~