I don't know that there are lockers at Kennedy Space Center, but I seriously doubt they would have them. I say that because there used to be lockers at airports, train and bus stations but they were removed for safety reasons.
I do have a suggestion for you though. I forget the name of it,perhaps it is luggage express, but it is a service with a charge of about $20 per person where your luggage is picked up the last night of the cruise and it is then taken to the airport for you. You are given luggage tags to attach to your luggage, but you don't see it again until you get to your final destination.
We have used it previously and it was worth every penny to not have to carry luggage around all day. Orlando airport is pretty busy on Sunday afternoons so, to me, this is the way to go.