We just did it in june, we did it thru the ships excursion, but as it turned out some of us bought the excursion before we sailed and others decided at the last minute and they were sold out. So they just walked to the end of the pier, took the ferry to st. johns, took a taxi to the bay and went snorkeling with us anyway, they rent the equipment right there. They pretty much rode right beside us on a different boat or taxi the whole time and it cost them the same amount of money.
Trunk's bay was gorgeous, I have some pics in my webshots, i wasnt as impressed with the snorkeling as they made it out. Its listed as one of the top 10 spots in the world to snorkel and while we all thought the bay was gorgeous, we thought we had seen better stuff out in the middle of the ocean.
It is right on the beach, you put your snorkeling gear on and swim right out and if someone is waiting on the beach they can pretty much watch you the whole time.
Let me know if you have any other questions. I dont mean it to sound like it was bad, it wasnt, it was great, I think I just expected more because like I said it was listed in the top 10.