Metro from Rossi to Cais Do Sodre, then from there you can travel by rail (separate ticket) to Alcantara-Mar, taking the Southern exit at the station it's a 100m walk to the start of the marina area. You can take a taxi too, there seems to always be a couple waiting there to take you back to the city centre. Or, you can carry on in the train to Belem, get some custard tarts and walk it off back to the Docas along the river. Maybe a mile and a half on a nice wide path.