I have nothing but good to say about Captain Marvin's. They are located across the street from the tender port (less than a block away--veer to the left as you cross the street). They offer several island tours and snorkeling options of varying lengths. They do require a credit card to secure your reservation, but charge it ONLY if your ship tenders but you don't show up. If the ship can't get in, there's no charge. When you check in, you can pay in cash, tell them to bill the card, or whatever.
Another great place is Boatswain's. This is where the turtle farm is on the island tour, but there's a lot of other stuff to do--you can make a day of it there if you'd like.
My description of it is stickied in the FAQ Excursion section. 
