I'd say if you're not interested in snorkeling, but just want to hang out at the beach, then go to Magens Bay. It's a bigger beach than Coki (but also more crowded), and is great for just lounging around. There's a bar/restaurant, chair rentals, etc., and it's an easy cab ride to and from the cruise ship pier.
On St. Maarten, I'd say try either Orient Bay or Little Bay for the same type of beach day, although you can get the same thing at Great Bay beach, which is closer to the ship. Great Bay is a little less relaxing, I think, since it's right next to downtown Philipsburg and has more traffic in the bay. The big plus with Great Bay beach is that you can take the water taxi to and from the cruise ship pier, so you can head back to the ship fairly quickly if you forgot something, or if you want to grab some lunch.
