Kris, based on past experiences, you could be on as early as 11:45am. But usually it's 12:15 or after. Carnival lists embarkation as beginning at 1:30, but as many of you know, people begin showing up as early as 9-10am.
The earlier you arrive at the pier, the earlier you can board the ship. Embarkation will also be smoother and faster.
Hope this helps. CD