I would say to absolutely fly in the night before. Flight delays happen all of the time & I would be too stressed out if I was flying in the same day as my cruise departed.
We did this last year & rented a car, which is much cheaper than Disney transportation. We stayed at the Sheraton in Cocoa Beach, because I had points for a free night. I think many on here stay at the Radisson.
I think the best way would be to do a one-way rental car, which can be dropped off near the port & they will shuttle you to the port. Of course you would want to drop off the rest of your party & luggage prior to doing this.
My family will be flying in the day before our cruise and stay at a hotel near the port. I had also originally planned to arrange a weekly car rental and do a stay and park/cruise hotel package, but hadn't thought of doing one-way day rentals instead. Sounds almost as easy and much more cost effective.
Thanks for the great tip!