I only know it from RCCL and Celebrity. You can check in at noon, but cannot go to your stateroom.
On the last cruise the hallways to the cabins were blocked and we were allowed to got in at 1 pm.
But no problems, we had lunch in the Selfservice-Restaurant.
Marita
Early is a relative thing. They usually won't accept new luggage until the old luggage is off-loaded and people have picked it up. So in other words you can't swing by at 7am and drop off the luggage. It could easily get lost or end up on a return flight to somewhere.
But - if you show up around 10am you will probably see the porters ready to start accepting luggage. The downside to this is that if you show up that early you will have a lengthy wait until the ship is ready to board.
I got there shortly after 11:00 AM and checked in my baggage. When I arrived there were about 100 or so people were ahead of me in line. We started the check in at Noon although they said it would be 1:00.
I was in my cabin by 1:10 and having linch on deck by 1:30