My wife and I just returned from a week in St Maarten. I spent dozens of hours researching and planning what to do, where to go, where to eat, etc... I ran across a review on Tripadvisor that said one could go to La Samanna for the day, rent chairs and an umbrella, and view the sunset from their bar. We drove to La Samanna (which isn't the easiest place to get to to start with) and was turned back by the security guard.
Now, La Samanna is on Baie Longue and all of the beaches in St Martin are required to be open to the public. So, we drove around to the public entrance and walked down to La Samanna. The walk on the beach is a very difficult half mile. Once we got to the Hotel, I inquired about renting some chairs and an umbrella. I was told they do not generally do this, but perhaps could make an exception (especially since there was literally NO ONE in ANY of their 115 chairs). Oliviere was very gracious in making sure we were comfortable, even bringing out a few fresh fruit skewers for us. We ordered from their beach / bar menu and were treated like one of the paying guests. Thanks Oliviere!
If you are planning to give this a try, I would check with the Hotel first.