The Cais da Ribeira or the riverside wharf as it translates has always been a bustling hive of activity. In the olden days it was where farm produce and wine from the Douro Valley would have been brought into the city. These days it is no less busy but the industry is tourism.
This was fairly early in the day, but it gets a lot busier. It's an amazing spot to explore and even get something to eat, although you will pay more for the privilege than most places in Porto.