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.
Remember, even if you board early your stateroom may not be available.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014
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.