Trunk Bay is nice, but not so much nicer to be worth the round trip ferry ride and cab rides.
You have nice snorkelling right off the beach at Saphire and a much larger choice of food and drink along the beach. Coki Baech is also a nice choice.
i love the ferry ride over to St. John's Island, there is so much to see on the way over. Then the cab ride to the beaches is just gorgeous, there is a scenic lookout that the cabbies stop at with a breath-taking view of the harbor.
There are lots of other beaches at St. John besides Trunk. Trunk Bay Beach is the stopping point of all the ships excursions so it gets very crowded.