Eileen - I have not eaten there, but I would guess that the Met in the Metropole Hotel would have good food. It all depends on what you were wanting. I believe that it is in the hotel and not sitting directly on the lagoon. The Metropole sits along the lagoon on Riva degli Schiavoni but there is an lot a walkway between the hotel and the actual water. The place does look lovely however.
If you were thinking of sitting at a table at the edge of the Grand Canal and watching the world go by while you eat, you might want to take a look at the restaurant in the Westin Europa and Regina. It is on the canal and right across from Salute. Once again I haven't actually eaten there but the setting is nice.
http://www.starwoodhotels.com/westin/property/dining/index.html?propertyID=75
There is also a very small Osteria tucked in by the Academia bridge on the Academia side that has tables very close to the Grand Canal. They do mostly pizza for lunch but had a more normal dinner menu.
there are also several places in Dorsoduro on the Zattere that offer canal seating near the Guideca canal. We ate at the one in front of the Pensione Calcina that was a bit hit and miss. Some things were great but others were ok. But the view and ambiance were wonderful. Great view of the sunset.
You might want to take advantage of canal seating for lunch before it is dark and do dinner inside somewhere.