We sailed on the Radiance of the Seas out of Seward last August. We drove. Our assigned check in time was 1pm. We arrived very early and dropped off our checked luggage around 10:30am at a well labeled conex outside the terminal building, then parked our car away from the port and got a ride back around 11:45am. The cruise terminal employees waved us inside the building. It was EMPTY. They didn't care that our appointment was at 1pm. We checked in and were on the ship by noon.
For a few short hours, we had a 90,000 ton, fully staffed private yacht.
Lunch was ready and very relaxing without a crowd. Practically every seat in every lounge and bar on the ship was available. We got some photos on deck and the interior spaces, checked into our stateroom and tried the solarium pool and hot tub.
If you enjoy having the ship to yourself, show up as early as Royal will let you. Maybe 11am.