I'm like U.. and eager beaver to get onboard. On all the RCI cruises I've taken so far boarding has begun by 12:30 (almost on the nose). If for some reason the ship is delayed clearing customs and immigration from the previous week's sailing, it could be a bit later, but not likely.
After boarding, if u want, u can leave the ship and return prior to sailing.
haven't done that specific cruise but my experience with RCCL is that they are pretty effecient... my guess is 1:30 at the latest... I would usually advise not getting there too early because you may end up in line... but on the Vision and the Soverign... they had enough personnel to make embarkation smooth... Have a great cruise !!!!!!!! ..