We were there a few months ago. As you may know there are many islands. The area where st marks square is is just one. There are many gondolas for hire and they are 20-25 euros per person for what seemed like 15 30 minutes. Not so much a tour of anything but an experience. tours-Italy.com and viator.com may have what you are looking for. My experience with them is they are good but represent a markup on what you can arrange on your own. What you do get is piece of mind that you have a reservation and will get it accomplished. Perhaps your hotel could point you directly to a provider. We didn't do it as we only had 1 day is to take a boat across to other islands.