The Grace hotel is located between Town Hall and Wynyard stations, and as the train incurs the $15 pp airport fee, bringing the fare to about $20 pp, I suggest a taxi or Uber. It will be easier with your luggage and most likely around the same cost or just slightly more, depending on the time of day and traffic.
The tram on George St (one block east) will take you to Circular Quay easily when going to board.
Brisbane's port is a fair way out, a cab, or uber may be best there if you cannot organise a ship shorex.