Yes, there are cabs at the pier in St. Johns to take you to the beaches, in fact there are plenty, no problem. If I remember correctly the cab was around $5.00 each way and the cost of admission in to Trunk Bay was $4.00. As for the snorkeling, its usually very good and the beach is just picture postcard beautiful.
Remember to purchase an underwater camera at home before you go, it will be twice the price once you get there. I'm sure you love the island of St. Johns :-)