I took the ships excursion for a boat ride from Charlotte Amalia, St Thomas to St. Johns island to snorkel. The excursion was great. The boat holding about 150-200 people picked us up at the dock where the cruise ship was docked, then took us to Cuz Bay, St. Johns where we transferred to "jitneys" (open-air vehicles with bench seats and an over-head awning). The jitneys took us along the north coast of the island for a couple of photo stops then continued on to Trunk Bay. Trunk Bay is a U.S. National Park and run by the Park Service.
Snorkeling gear is included and the swim time is about 1 1/2 hours. Just off the beach is a small island with an underwater snorkeling path. It is marked with underwater signs to guide you along. The water debth is about 8-10 feet deep. The beach has nice fine white sand.
Services at the beach include a place to lock up your valrables in lockers, a snack stand, a shower and wash-off area and a small kiosk selling souveniors. I don't know the name of the vendor that provided the snorkeling service for Holland America Line, but is was great.
I prefer to book my excursions through the ship. Even though I pay more, the ship has evaluated the vendor and I am sure I am getting a reputable and safe excursion. Other people prefer to arrange their transportation themselves. This sometimes takes a bit longer to wait for scheduled boat service to St Johns. Whatever you do, research the excursion carefully. There are only so many vendors of excursions. Look at another cruise line's web site for details of other excursion at that port. They may offer you more information for available excursions than yu cruise lines brochure. You may just reserve an excursion off the ship. Holland America has a brief description of excursions on their web site. They send a more detailed booklet of excursions with the tickets. Enjoy you adventure.
Yes, there is decent snorkeling in St. John/Trunk bay, well worth the trip. You can also do this on your own if you want to take a cab from the pier to Red Hook ferry, and once at St. JOhn another shorter ride to Trunk Bay. They also rent snorkel equipment on the beach, but if you already have your own or may purchase some before you leave, thats also a good idea. St. John is a beautiful island, you'll love it!
Last year we did the America's cup regetta in St. Maarten and that was a lot of fun, would also recommend that too.
I have just received my excursion book for my August cruise and there is more information in it than on the website. But I also went to the RCCL website and read their description of basically the same tours.
We have decided to do the kayak ecotour on St. Thomas and then on St. Maarten we are not taking an excursion, just going to the local beach. We had a week's vacation in St John a few years ago. I highly recommend going there, it is a beautiful island and the snorkeling at Trunk Bay was excellent.