On my cruise documents, it states that the earliest boarding is 12:30 and the latest is 2:30. In years past, I remember getting on the ship around 10:30, has this changed or do people still board earlier than 12:30? We are sailing the Glory in October.
I think this is a general guideline which allows them to fully clean, stock, and refuel the ship. It is possible they may board early. It is just like any other form of commercial mass transportation in this regard. Don't count on earlier, but you may get lucky.