When you post about St Martin/ Maarten, everyone will point you to Saba. Ok, that's understandable, and I second Cecil's comments above.
That said, St Martin's offerings are downplayed a whole lot. Last July, we spent a week on St Martin and managed to take two 2-tank trips there. The dive op was wonderful, the waters were fabulous, and the sea life was excellent. The reefs seem "young" to me, and they have a new "wreck" (the Porpoise... I think it's called, I'm sure they'll take you there) which delivers a swim through I heard was really nice. It's true that St Martin's diving isn't Bonaire but we didn't expect it to be. It certainly was not a waste of money time or effort, either. I was pleasantly surprised.
Pick your dive op's location carefully... remember the ones in Philipsburg will be full of cruise ship passengers (not that there's anything wrong with that) and will plan their trips accordingly (that's the problem). We were staying in the Terres Basses, so we dove out of Simpson Bay. Distances to travel by car take some time, as you'll quickly figure out.
You'll love the island. Have a fabulous trip!