While visiting Rome we discovered from the locals that there are water fountains all around that you can drink from. In Venice, they have these same fountains with fresh cold water. Bring a water bottle and fill up your bottle when need be. It saves money since Venice is expensive and it reduces plastic waste.
DON'T FEED PIGEONS
It's kind of the same advice I gave with the monkeys in Asia. Once you give them little a little bit of your food, more will come and they won't leave you alone.
BE CAREFUL WHERE YOU SIT
Since the best way of getting around is walking, you're going to want to sit at some point, but you'll find there the options are very slim. There aren't many benches. The local council has a number of rules for visitors in order to maintain its beauty, reduce litter, and preserve a modicum of peace/sanity for residents and if broken, you could get a 200 Euro fine therefore you can not sit just anywhere you want, even if it's a step.
If you thought that finding somewhere to sit was hard, bathrooms are even harder. Public toilets are few and far between in Venice, they have limited opening hours and cost 1.5 Euro to enter. You'll see signs around the city that points towards the bathrooms but you'll find yourself walking for quite a while without finding them in the end since they are tucked away.
What we found to be the best solution is to go to a cafe, order an espresso or a soft drink, use their bathroom, finish your drink, and then leave. This is acceptable just try not to make it too obvious.
BEAT THE CROWDS
WAKE UP EARLY! This is the best advice we could give you.
Venice is one of the most beautiful cities we've been to, but in the afternoon it quickly losses from its beauty with the number of people walking around and trying to capture photos in the popular spots in the city.
6 AM is a great hour to start your day before everyone arrives. You'll start feeling the crowds already at 11 AM.
We found it pretty hard to find a sim card in Venice. Try to come with a SIM card already on you or before you arrive in Venice. The only spot we ended up finding was in Vodafone near Rialto Bridge.
SITTING FEES IN RESTAURANTS:
Do take into consideration that in restaurants they charge sitting fee which is usually 2 Euro per person.