HAL's Prinsendam is the only cruise I know of that will be taking that repositioning voyage. Celebrity's Galaxy sails from Baltimore. Most of the other lines use a more southern route, leaving from Miami, Tampa, or Ft. Lauderdale. If you can catch a cheap flight to Florida, you may want to consider that option!
My parents took a transatlantic cruise (stopped in Madeira, Morocco, can't remember where else), and got off in Barcelona, then rented a car and drove to Portugal. They got to cruise, see a bit of Spain, and spend another week or two in Portugal with family before flying back to Canada. They saw it as a free trip to Portugal.