poule, venice is notoriously short of public toilets. The ones that do exist can be accessed by paying cash, so keep plenty of coins in your pocket (and it's not cheap). but yes, you can access the toilets for free using the Venice Connected card (www.veniceconnected.com) which also gets you cheap transport and even museum entry.
make use of the toilet at your hotel at the start of everyday, and when you stop for lunch make use of their facilities.
Here's a link to a map of public toilets in venice
Phil about 6 years ago