We just returned from the Dawn yesterday. On Friday we were on St John at beautiful Trunk Bay. We did the (St. John Beach and Snorkel) tour. This is incredible. Trunk Bay is a U. S. National Park Wildlife Reserve. The beach is pristine fine white sand with beautiful crystal clear water. There is a lifeguard and snorkel equipment/snorkel instructors on hand to assist. This site also has a marked underwater snorkel trail located within a buoyed area. You will see lots of colorful fish and beautiful coral. Showers/changing rooms are available as well. We found it especially nice to be able to swim and relax without anyone bothering us. After traveling from LA through the Mexican Riviera,Costa Rica and the Panama Canal to the ports of Cartagena,Aruba,Barbados, Dominica, and St. Thomas, (where we were always beseiged by peddlers, hawkers, and taxi drivers), it was a wonderful relief.
I will definitely recommend this trip. If you book it through the ship, you will have few hassles and a beautiful and rewarding day. It will be a highlight to your trip.