We only checked in in Miami and Jacksonville.
But in both harbors it was possible to check in earlier. We checked in once at noon and one at 1 pm.
Maybe the stateroom is not ready for you, so take some things, you need for the first hours, in your handluggage and enjoy discovering the ship, until your stateroom is ready.
Even your luggage might not be there, but this has nothing to do with the early check-in.
So be prepared, that you have everything you need for the first hours in your handluggage.