My wife and I honeymooned in St. Martin this past June. We stayed at the Oyster Bay Beach Resort (now known as the Radisson at Oyster Bay, I believe). We hooked up with a bluewater guide service through the activities director at the resort, and we were able to get some bonito and yellowtail, but the highlight of the outing was connecting with a couple of wahoo on a fly and actually landing one.
As far as bonefish and tarpon are concerned, we weren't able to find any outfitters, but we didn't look too hard either (after all, it was a honeymoon
). If you put the keywords Saint Martin and fishing into a search engine, you'll come up with some connections. That's how I was looking before we made our trip through the service at the resort.
Good luck to you.