The ships generally dock between 5:30 and 6:00am. Disembarkation begins when Customs has cleared the ship, usually at about 7:30am. If you have early dinner seating, you have breakfast at 6:45am and the ship is usually cleared shortly afterward. Claiming your luggage is pretty easy and it doesn't take too long to get through the Customs line and out of the terminal. Everyone must be off the ship by 9:00am.
The ride from the port to MCO takes about 45 minutes to an hour. Using the
DCL buses, I'm usually through airport security by 10:00am. I'd say that any flight after 11:00am
should be fine. After noon, there would almost definitely not be a problem. The ship is sometimes delayed getting into port. It doesn't happen often, but be aware that it could happen, probably fewer than 1 time in 10.
The trade-off is how long you're willing to wait at the airport if the ship is on time vs. how certain you'd like to be that you won't miss your plane. Our flights are almost always at 12:45pm, but that turns out to be the best we can do. I personally would be comfortable with anything after 12 noon, but I think this is erring on the side of caution, as waiting in the airport terminal doesn't particularly bother me.