Breakfast in Venice Hotels can be very variable, depending on the Hotel .
At one end of the scale it can be a very basic Italian or "Continental" breakfast - coffee and rolls, perhaps with a litle pot of jam. Quite why it is called Continental escapes understanding - such meagre breakfasts are far from being the rule in the Continent of Europe.
At the other end of the scale you may have a choice of a cooked breakfast or a cold table with a wide range of cold meats, smoked salmon etc. But you can never be sure until you see it.
One alternative is to go out to a local bar and have a good coffee and a choice of croissants, panini, tramezzini etc..
Another choice - Right off of the Rialto Bridge you will find a wonderful fresh fruit, vegetable and fish market. Go to the fruit market and pick your favorite fresh fruit for breakfast and then find a quiet nook overlooking the Grand Canal to enjoy your breakfast.

