We did the Buck Island Snorkel and it was nice. A nice boat ride, cerulean water, sunshine, it was beautiful. Snorkeling was around an hour and the reef was 95% dead. No colourful coral, but some pretty fish. Probably not worth 100 USD per person and probably not the best snorkeling you will do in your life. The island isn't very nice and you feel kind of guilty driving through the slums on an air conditioned coach.
Beautiful Island, nice foliage, and very nice beaches. We went to the Marriott for the day and enjoyed lounging around paradise. I'd go back in a heartbeat.
We took the water taxi to Grand Anse Beach. It's quite nice but filled with people trying to sell you things. It's lovely, but after two hours, you're ready to leave.
Screen saver worthy beach, but filled with tourists. We were cautioned not to go to Magens Bay due to the lack of facilities, but in hindsight, it would've been nice to get away from the crowds.