Totally agree with what nacho mentioned...pick up the car the previous day. It won't cost any more.
A couple other things you may want to price out: How are you getting back to the airport after the cruise? If renting another car, look into keeping the car parked at the port and it'll be there when you return from the cruise. Of course, also add in parking at the port. This sounds expensive, and likely is, but its worth a look. Another option, that we used, was to get the car at MCO, use it at Disney, then drive to and leave it at the port during the cruise, then return it to MCO when you go home. This actually worked out less expensive for us one time, even with port parking added in. The longer period kicked us into a weekly priced rental, for which we also had codes and coupons. I think its pretty rare this works, but its worth checking.
Also: a nice thing about keeping the car at the port is you can leave stuff in the car that you don't need on the cruise (i.e., Disney souvenirs, laundry, etc.)