Hi merjo, welcome to Cruise-Chat. We've been to St. Kitts a few times. The first time we did a tour to the fort and a batik factory, it was really interesting. The last few, we did catamaran sail/snorkel trips through the ship. I have heard good things about Turtle Beach, but can't give you first hand knowledge because we didn't go there. The snorkeling we have done there has been pretty good.
This might give you some ideas. Hope you enjoy your cruise and once again, welcome aboard!