In my three trips, I still have only explored about 10 beaches.
Maho- Great for watching plane landings and take-offs, fun for swimming, some waves, but not too hard to swim in.
Cupocuoy- White sand, rocky, large waves, nudity is allowed, great romantic beach.
Frier's Bay- Has a sandbar that extends out a ways, making it great for young children, great family beach and couple beach bars as well.
Pinel Island- Nudity on top allowed, very overpriced to rent a chair and umbrella - $18 a day. The snorkeling was great, however. The ferry over was only $6 round trip.
Orient Beach- Great variety of activities, from boats to trampolines, parasailing, jet skis, etc... Many beach bars, and nudity is allowed. Very long beach, very populated. Waves are perfect for playing in.
Dawn Beach- Good for bodysurfing and has a great beach bar; chairs and umbrellas are available for a cost. There is currently construction going on in the back, so view is not so great right now.
Guana Bay- Great hideaway and very large waves, bodysurfing beach only. Waves come in large and fast. Great beach bar and restaurant called the Boardroom. Great for steaks, shrimp, and local food at a much lower cost than Grand Case and Orient Bay.
This ended up being our favorite hangout this visit. It has showers, bathrooms, billiards, indoor and outdoor seating, and is not well known, so is great for romance and to get away from the crowd.
Little Bay- Pretty good snorkeling off to the left of the inlet and great for families, no waves, very calm.