That would probably be fine but you'll probably wait around for a little while. We arrived around 11am for the 8/5 cruise, there are a few lines you have to go through first, then you wait for them to start boarding. Boarding time varies but for us it started around 12:30 - 1pm.
Carnival Spirit - Exotic Mexican Riviera - January 9, 2008
Carnival Pride - Mexican Riviera - August 5, 2007