There are plenty of great beaches in St. Maarten. Mullet Bay is fantastic if you are there during the week because the beach isn't crowded at all and the water is just unbelievable. You can order ribs etc... on the beach too.
Another great beach is Tintamarre island, a small island just off St. Maarten on the French side. That place is just amazing too. You will have to use an excursion to get there... Last time I was in St. Maarten, we used the Golden Eagle Catamaran tour to get to Tintamarre. The tour was great! It lasted four hours and we were back on island for the other half of the day.
A good website to get info on the beaches on the island and the various tours is www.sxmdeals.com