At virtually any time of day you will see dozens of people squeezing together on the central parapets of the Rialto Bridge, taking photographs looking up and down the Grand Canal. I’ve done it myself. The balustrade is wide enough to lean on (or sit on if you’re feeling daring) and you can comfortably watch the world going about it’s business below you.
The trouble is, I don’t think the views themselves are that impressive unless you’re fortunate enough to capture a particularly interesting configuration of vessels on the waters below. Looking north your view is immediately curtailed by one of the bends in the canal, whilst looking south you have an abundance of palazzi on either side, there’s nothing that really draws the eye so well.
Much better to go to the Ponte dell’Accademia.