United normally does participate in EZCheck and it would be great for you. The Fort Lauderdale airport won't let you check bags until 4 hours before your flight, so when EZCheck isn't working I have to store bags ($10 each, and they search them) a half level down from right end of the ticketing desks. Then we go in to the United lounge until I can check luggage in. I leave the lounge, get the baggage tags, get the bags, drop them off and reenter security.
Have had two cruises recently where the luggage handling wasn't available, but we ALWAYS use it if it is. Ask at Guest Services early in the cruise for the form.
If it isn't available, you might want to rent a car for the day. Avis and Budget are just across 17th from the north port entrance, within walking distance if your ship is at the usual terminal. When you make your reservation specify airport dropoff. No dropoff charges in Florida.