It's hard to know what will happen, in this post-Irma age. Pre Irma, many places on the French side would be closed in September. Most places on the Dutch side stayed open. Now, there is not a whole lot open in Grand Case on the water side, so if you have your heart set on that, there is not a lot to pick from. We have often gone to SXM in September in the past, although not for a few years. If you can roll with it, and don't have your heart set on specific places to dine, you should be fine. Just know that you NEED to get trip insurance.