You can set up a scuba diving trip with the tour office. We went snorkling in a remote part of St. Kitts and it was quite nice, though not nearly long enough in my opinion.

As for nightlife, there is some in Basseterre (about 10 minutes from the Marriott) and there is live music some nights at the hotel. The Marriott is opening up a nightclub, but it isn't open yet. All in all, rather tame nightlife, but I was okay with that.