Hello JMWARD. We have done Paris on our own but on a different cruise line and we booked it through the ship. I just noticed that
DCL doesn't offer the same thing that we did on the other cruise line. That stinks! It was nice to be dropped of in the center of the city and then we had a few hours to go do the few things that we wanted to do, and then be driven back to the ship without any worry. (It was a bus ride, not a train)
Just looking quickly on line, there is a train that runs from Le Harve to Paris but the schedule doesn't seem ideal.... at least to me and I'm not sure where the train station is located. I wish I could offer you more info. Good luck