We took a cab to Dawn Beach, on the Dutch side of St. Maarten. Great swimming and snorkling. There's a little restaurant there with restrooms. You can rent chairs, umbrellas, floats, boogie boards, etc. from Wayne right there on the beach. The cab ride was about 15 minutes from the cruise ship pier, and cost $15 for two of us.
They may ask you if you want to tour the timeshare unit which is adjacent to the beach, but they didn't push the issue at all. We said, "No thanks" once, and were never bothered again.
Great Bay beach is also a good beach, but probably not as good for snorkeling. There's a water taxi that takes you to Philipsburg ($5/person for unlimited rides) from the pier, and the beach is right there. It's much bigger than Dawn beach, but there's a lot of water traffic in the bay, so the snorkeling isn't as good. For sunning/swimming, etc., it's great.