I agree that Ristorante Alfredo Sull'Arno is an excellent restaurant,  We walked by at lunch time and it was packed so we went back later and got a table by with a window view looking over the river.  We had a great meal.

This restaurant featuring typical Tuscan dishes sits on the southside of the Arno and has a beautiful view of the Ponte Vecchio.  In the summer they open their terrace allowing diners to sit outside and look out over the river and bridge while they eat.  the menu includes seafood, steaks, veal and pasta.  Ask for a table with a view.  The food and service are excellent.