Wow, where are you coming from that you can Arrive in Miami at 7 AM? As Sonny says, I wouldn't want to arrive at the port any earlier than 10AM because its rare that the can begin boarding earlier than 11:30AM and you are just going to be sitting/standing around. In fact, if you arrived at 8, you are going to be caught in the rush of people coming off the previous cruises!.
I'm not even sure they begin accepting luggage for a cruise that early or I would suggest you go and drop it off and then have the cab driver drop you off at Bay Side and you could have breakfast there and go back (but that would cost another cab fare of about $15 -they charge an entrance fee to the cab which pass it on to you)
IF you have nowhere else to go but the port, I would suggest you hang at the airport and have breakfast before heading to the ship.
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