We just returned from a cruise that took us to St Martin in mid Feb. We toured the whole island and spent time on several beaches. If I where going back and wanted what you described I would spend some time at Mullet Bay beach. I has great sand, shade behind and lots of different things to do. Not a watersport rental kind of place, but calm over to the right side for young kids to play. Waves on the left so body boarding. Reef to snorkel on and sand volleyball. It was beautiful the day we went there. As for as nude or topless sunbathers, you may run in to them just about everywhere. Orient beach is the most famous and they where everywhere on that beach, but we did we alittle bit on all beaches.