electech gave you a fine answer. But I have a couple of details to add just so you know.
The cruise docs will say that you can start boarding at 1PM to and should be on the ship bby 4PM. Meaning the ship is scheduled to depart at 5PM. Most of the time you can actually start boarding by 12PM. So if you schedule you flight in the morning you should be ok. Even if you get a 2 hour delay, you would still be ok. But, you won't have time to bother finding something between the flight and the ship. Even if you manage to get into Miami airport by 9AM, by the time you get your luggage get through the taxi line (you implied you are doing your own transfer to the ship) and get from the airport to the ship, you will at best case arrive at the ship by 10AM more likely 10:30. So you stand their staring at your ship for an hour and a half before you get on.. big deal. Its not worth the hassle and expense (the taxi are charged a port entry fee which the pass on to you, so going into the port and leaving again will cost you $25 plus the cab fare) to go to the Bayside shopping mall. I always try to get on the ship as early as possible because once you get on, your are cruising! (Lunch is ready, you can explore the ship, maybe go see your cabin etc etc)
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007