Rhen - we've been twice and this is what we did while there:
1st visit: We took the local bus over to the French side to Marigot, changed buses and then went up to Grand Case where we ate at a local bbq place and then swam at a beach right behind there. The bus was a total of $2.50 to get there and $2.50 to get back...p/p. The food, ribs, slaw & bread was $4.00. The beach was free.
2nd visit: We went to the French side and caught the ferry over to Anguilla and then went to a beach that had a restaurant, bar and a wonderful beach. You will have taxi or bus $ to the French side, Ferry $ to Anguilla and I think a tax from the French side...then a taxi to and from the beach, drinks and food there. This day was quite expensive for the time you have to actually hang on the beach. But, it was worth doing one time... plus we had wonderful traveling mates with us.
Enjoy St. Martin / St. Maarten!!!