Jimão Tapas e Vinhos at Praça da Ribeira has excellent food at reasonable prices. I was seated upstairs where there were big windows that might look out on the river, but I don't actually remember if there was a river view. I know you can see the restaurant from the promenade but it is tucked slightly off the promenade so it may not have the view. Reviews just praise the food.
Alternatively, on the Gaia side of the river, there are a number of restaurants with a view of the bridge and Porto. Taberninha do Manel on the Gaia riverfront was recommended, but it was closed the day I was there.