In the Rialto Bridge area consider Trattoria Da Marco Valla Fava on Calle Stagneri. It is definitely pricy so you might want to do lunch there rather than dinner. Some of the best sea food ever!! Try to eat inside rather than outside. The place has been around at least 50 years so they must be doing it right. Also in the Rialto area (on the other side of the Grand Canal) when walking to Piazzale Roma is an out of the way place named Pane Vino e San Daniele on Calle Dei Boteri. Definitely a hole in the wall local place, good food, good house wine, and not expensive at all.
Canal side places are usually overpriced and under quality. Especially in the Rialto Bridge area. There is an exception. You might want to try Trattoria Da Giorgia ai Greci. It is very close to San Marco, but well away from the crowds. A bit pricey, but most places in Venice are, and the food is really great.
Google maps street view is a great tool to locate these places and "walk" to them ahead of time.