Matoshinhos, the district where your cruise ship will dock, is very well connected to Porto city center by metro. Actually it's a fast tram line managed by Metro do Porto. It’s a fast and cheap way to get downtown. Metro’s run frequently almost every 10 to 15 minutes and a one-way ticket will cost less then €2 euro per person for a 25 minute ride. Take a metro direction Estadio do Dragao and get off either at Casa da Musica or Trindade, which is very close to Porto historic center. Here you can find more info about the metro from Leixoes to Porto.
How do I get to the metro station from Leixoes (Porto) cruise port?
This depends on where exactly you will dock in Leixoes cruise port.
If your ship docks at the cruise terminal dock nr.1, the closest metro station is Mercado. Exit the gate of the pier and go to the right. A couple of hundred meters away you will already see a drawbridge. Cross it. Down on the other side of the dock you will find Mercado metro (tram) station.
If you dock close to Matosinhos Beach, exit the docking area and go to the right. At the roundabout, where you will also find the Matosinhos Tourist Office, go up east into the Avenida Da Republica, cross the next roundabout with the statue, and you will end up at the Matosinhos Sul metro (tram) station.