This is not an easy question, which I believe explains why no one has responded. Venice is not particularly known for excellent food---and if you will find it the price will likely be quite high. Most restaurants cater to tourists who want a "Venetian experience" without the high price. For this reason, most canal side restaurants, particularly those near the Rialto Bridge, are to be avoided.
However, Giudecca is well outside the beaten tourist path, so for waterside dining with an amazing view you could consider Cip's Club at Hotel Cipriani.
http://www.belmond.com/hotel-cipriani-venice/cipriani_restaurants
This is one of Venice's finest hotels and Cip's Club is their "casual" restaurant. The outside seating is on a deck with views across the Bacino to San Marco. (This suggestion presumes you will be there Innthe season for outdoor dining, which generally begins mid-April). Request a waterside table when you reserve, and mention that it is your anniversary. They may pick you up at their San Marco private dock for a ride across the water, which would certainly add to the experience. Otherwise you would take the vaporetto.
Another possibility, if you want really good seafood in an authentic Venetian setting ( not canal side), would be Osteria Alle Testiere, a tiny little place up near Campo Santa Maria Formosa. They have two sittings for dinner, at 7 and 9:30. The earlier one is mostly visitors and the later mostly Venetians. You need to call to make a reservation,mat least two weeks in advance. This place was incur neighborhood when we spent a month innVenice and we were unable to get in as we did not reserve far enough ahead.
Other places I have seen recommended by locals ( but which we have not tried) include Il Ridotto, Alle Corone, la Zucca ( small and unpretentious,mood food), la Colombina, A Beccafico Arte ( Cannareggio), and A Beccafico ( outdoor dining on Campo San Stefano).