Tobago has Pigeon Point Beach. Take a cab (20 dollars pp). Rent a lounge chair for 2 dollars. Nice beach and facilities. There is an entrance fee that was about to change so I can't comment on that expense. The shore excursion from the ship was more then double the actual cost. If you take a cab go about an hour before the shore excursion. You will get a better spot on the beach. It was a very popular excursion. Anyone that showed up after the passengers did not get a lounge chair.
St. Martin has great bars and not too bad of a beach. You can get a lounge chair and a bucket of five beers for 15 to 20 dollars. The retail vendors are way to aggressive