Mullet Bay
Mullet Bay is a white crescent tucked away in a less trafficked part of St. Maarten. It's far from the main tourist areas, so you won't see too many cruise ship visitors here. It has an active surf that beckons surfers, plenty of shade along the shore, and a few onsite amenities.
Food and drinks are available from a number of stands, and prices are very reasonable. There are public restrooms available near the center of the beach. There are chairs and umbrellas for rent on the beach as well, but the sand is soft and there are lots of spots in the shade of the palm trees that line the sand.
The surf here can be very exciting - it's not unusual to be standing in 4 ft deep water and suddenly have a wave come over your head. Children and children-at-heart are both found frolicking in the water nonetheless, but definitely be aware that the water is a bit wild and keep an eye on the kids. Surfers often catch a few waves on the left side of the beach.
There are a number of rocky areas that may offer some snorkeling opportunities, but more often than not the choppy seas would make that a dubious endeavor. It's probably a better plan to stick to playing in the water and hit another beach for snorkeling.