Trunk bay is our family's favorite beach. We experienced it when in port at St. Thomas, we did a combination of cab/ferry from Red Hook/cab to the beach. It is a national park. There are clean restrooms (by caribbean standards), rental snorkeling equipment, concession/food available there. The beach is heaven on earth. There is a marked snorkeling trail that was pretty good. White sand all around it, no rocks getting in, no motorized water craft. Just Beach, Beach, and more Beach. We did this independently of the ship and were able to spend the day. The ship's people were summoned far earlier than we were and it was well worth the $$ spent on cabs and ferrys to do this. St. John is 90% undeveloped and looks like the caribbean should look. Amazing.
RCCL Nordic Empress 1993
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You should be porting in St. Thomas. From there you will need to take a cab to Red Hook (30minutes to 1 hour depending on traffic) to catch the ferry or you can catch the ferry from Charlotte Amalie. The ferry from Red Hook is about 20 minutes, the ferry from Charlotte is about 45 minutes. Once you get to St. John you take a cab to Trunk Bay (about 10 minutes). The one word for Trunk Bay is Awesome. Use your Find feature, in the upper left hand corner of your post, type in Trunk Bay and it will pull up hundreds of posts on it. Best wishes and have a great cruise!