Just came back from Venice and i'm already planning my trip back...the food...oh boy, the food was awesome and I managed to find a great seafood place... originally i was planning on eating a Fiaschetteria Toscana, but Susana, the owner of the Campiello Zen (the b&b where i stayed - awesome!) gave me great recommendations...for seafood she suggested Osteria La Bottega ai Promessi Sposi..about a 15 minute walk from the train station. The food was awesome, the prices were affordable and they had a a great menu...ok, now i'm sounding like that guys from one of the City Island seafood places, but the food was great and so so fresh...we had a the mussels and clams, which was superb, a plate of frito misto, so light and crunchy and the best of all dishes for me was the calamari in tomato sauce...soooooo tender and meaty, perfectly executed...i highly recommend the place, just make sure to reserve a table, it was packed when we visited.
Venice is difficult even if with a large budget. It requires a little luck and tempered expectations. Most seafood are cooked simply therefore, freshness/rarity of the seafood and care are the most important criteria. Expect to pay about 12-15E for a main plate at a nothing fancy place. With that, one will get good farmed branzino or orate, fresh calamari, seppia, bacala, etc. One would have to spend more for more exotic things uniquely Venice such as moleche, baby grey shrimp, canocia, scallops with roe or for fish such as wild branzino, tuna, monkfish, swordfish, St. Pietro.
I would try Da Alberta, Al Ponte (there are many in Venice but the one in San Polo is also called A La Palantina), Vivaldi, Antico Dolo, all in San Polo; in Cannaregio, la Bomba, alla Frasca; Bessetta in San Croce; Dona Onesta in Dorsoduro.
We spend at least a month or two in Venice every year (just returned after 5 weeks) and when we want to eat out, we tend to skip the medium price places because we can cook similar food in our apartment. We tend to either splurge, go for ciccheti or pizza.