The tenders run back and forth the whoel time the ship is there. So you can get back to the ship anytime you choose. If you are only planning to take a trip to 7 mile beach, then I wouldn't even bother with a paid excursion. Take the tender in, grab a taxi and have them take you to a place that you can have access to the beach. They have plenty of resorts and restaurants that will rent you beach chairs and have facilities you can use. But, if you want to stick with the ship's excursion, that's fine too. I don't think you will have to worry about booking it in advance as those don't tend to sell out unless its a combined excursion (like horseback riding and then beach break or sailing/beach break... etc)
The tender process is really simple and it does not take long to get on and off. No longer than say a bus would take to load up and drop off. Your longest wait will probably be waiting for a return to the ship tender as you have to wait for the next on to come in.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013