Hi Joy,
We once did an excursion on the "wild thing" that landed us at Trunk Bay, but they don't always go there, we were very lucky that day. We've also just taken the Red Hook Ferry there on our own, its really very easy to do yourself. I think as long as you get a chance to snorkel over there, either one would be good. Its definitely worth the time and effort to get over to St. Johns, very beautiful island, so go for it!