My DH can't swim, either, and we've taken this excursion several times. The ferry is boarded/unboarded right off a dock on both ends, and the safari taxi ride is kind of like a canopy-topped school bus. It has a mix of railings and solid sides, but i would make sure the child sits on the "inside" seat, just in case. On the
DCL sponsored tour, everyone gets a life vest and snorkel gear provided. There's a safety talk and demo you must take by park service people before you are allowed in the water. You are also supposed to have a swim buddy before entering the water. It is a bit over-regulated for me, but will probably suit your family just fine! Note...due to the amount of time it takes to get to St. John's (45 min) and then to Trunk Bay (20 min) and then back again, you end up having less than two hours at the beach. It has good restrooms, a small gift shop, and small concession stand with beer and soda. Enjoy!