Unless you're a diver (or a banker) I'm not sure there's really much to do there but stroll downtown (estimated time needed - 1 hour). We walked, drank some fresh coconut water, ate some conch chowder and went back to the ship.
I rarely book excusions from the cruise line, but this was a fair deal and I was really glad I did it. Not only was our local guide wonderful, but one of the cruise staff came along to insure everything went smoothly. Although the cruise agent had a clipboard with tons of info to report back to the cruise line, she also took part in all the activities and was really 'one of us' during the whole experience. It was nice to know that MSC cares so much about the quality of excursions that they send agents along to insure the product delivered is exceptional. Well done.