I am wondering if it is difficult to find local markets to get water, snacks and incidentals in Venice? We are staying in a AirB&B right in the heart of venice and will be traveling with two teenage boys with hallow legs - they are always hungry!
There are a number of supermarkets in Venice where you can shop. If you tell us where your rental is located, we can give you specifics.
There are lots and lots of bars, snack bars, etc., that have counters at which you order the food that you point to and choose (under glass) and then take it away. Sandwiches, veggies, cichetti of various types (some are open-faced meat/cheese/seafood/veggie combinations on top of bread).... You won't walk far without passing such a bar/cafe/snack bar, in much of the city.
There are several COOP supermarkets in Venice.
Where do you mean by "right in the heart"? Most of Venice is what I would call that. With a Campo or at least a vaporetto stop or at worst a sestiere we can give a specific answer which fits you better than a general one.
https://www.google.com/maps will show you supermarkets if you ask it. Use the "Nearby" link.
Not clear where you are staying, but as others have mentioned their are plenty of snack bars, & Coop Markets everywhere. We were at the Rialto Market yesterday and their were plenty of choices. We stopped by a snack bar for a quick sandwich. It was a bargain at less than €4 for both of us! Wonderful healthy fruit stands to enjoy as well. We pick up water along the way each day. Enjoy your stay!
I assume you mean grocery type stores? Yes, there are plenty. Search google maps for “coop Venice”. Coop is a popular grocery store chain and there are plenty around the core of Venice. There are other stores as well, that’s just one example.
You can pick up bottled water and simple snacks at all sorts of shops - coffee bars with a drink cooler, small gift shops that also sell granola bars, kiosks, pizza take away windows, etc. You pretty much can’t miss these. Grocery stores are easily available but you can’t count on just stumbling across one. So it’s good to look it up ahead of time.
In addition to COOP stores, there are also several CONAD grocery stores in Venice which usually show up on Google maps. If you want nice fresh fruit and vegetables at good prices, go to the Rialto market but get there well before noon.