You will be able to book on board - be prepared to still be at the airport in the early afternoon though, so a long wait ahead. You don't say how many people but Larry's Limo will do a charter for the day that will take you on an Everglades tour, out shopping, out to eat and end with bringing you to the airport. I am also a great advocate of hiring a car and DIY and then leaving the car at the airport.
If you 'Google' 'Everglades tours' there is so many to choose from you won't know where to start. Good Luck. Wilba P.S. Don't be suprised if they are a pile of money cheaper than the ships offerings!
They do close internet booking as you get close to the sailing date but as you have not said the date or even the ship it is difficult to say. Wilba's suggestion sounds good provided you can arrange for your cases to go with you. .... Neil.
Yes I did wonder about the cases......Mmm .