My opinion for what its worth is fly to Orlando. You can get much better rates Orlando than you will to Melborne. Getting a Shuttle from the Airport to the boat is not hard at all. Infact I know Carnival has one. We took it last week from the Airport and the ride wasnt bad at all.
If you can fly on Jet Blue or Delta Song they have great rates into Orlando. I think I paid like 140.00 round trip from Boston to Orlando on Song. Sorry I dont remember the exact price as I booked awhile ago.