We did the Southern Caribbean this past April (2017). In Saint Kitts, we just booked a taxi to the Carambola Beach club. It was a super clean beach, super clean restroom facilities. There is a restaurant and bar on site and you can rent beach chairs & umbrellas. We stayed most of the day and had fun building sand castles and swimming. The water is super warm and stays shallows for quite a distance.
In Martinque, we just stayed on board to relax. In Antigua (our favorite stop of the trip), we did the Reef Riders excursion. You can book it through Disney or book it on your own. It was a lot of fun with a snorkling stop... the reef was pretty shallow and the waters calm (good snorkeling). The people who run the reef riders shop were the friendliest people we met the entire trip! Shout out to Tygger and Anthony! There was an 80 year old lady whose husband had to back out of the excursion and she came anyway. They were so patient and kind with helping her with the reef riders and snorkeling. Anyone who was new to snorkeling, they were more that willing to help. They had amazing equipment and the reef riders were just FUN! My in-laws did the Lobster Lunch & Catamaran excursion in Antigua and just LOVED it. I think it was about a 6 hour excursion but was a lovely catamaran ride, a stop at a beach or two, a snorkeling stop and a fresh grilled lobster lunch on board a long with open bar.
Enjoy your cruise! We would love to do the Southern Caribbean again!