Hi zaf88,
For dive shops on St Martin, especially as you're looking for a small op, I highly recommend Octopus Diving in Grand Case (located on the main drag, Boulevard de Grand Case, in Grand Case on the French side of St Martin), about 10 miles (16 k) from you at the Westin. Their website is
www.octopusdiving.com. The owners are Chris and Sally. They book 6 people max on the boat, so it's always small groups with lots of personal attention-no cattle-boat experiences there. Which also makes it good for certification or other courses- not sure if you're taking any? Anyway, I've been diving and snorkeling with Octopus many times over the last 3-4 years, including 3 dive trips (20 dives) this year alone.
Safe, professional, personable staff, well-maintained equipment. And diving with them is also fun and relaxing. It's the best service I've experienced at any dive shop- possibly b/c of the small size, more probably also b/c of the people....the people make the shop, and this is no exception here. The divemasters know the sites and the creatures very well...wrecks, reefs, and "secret" sites that you don't see on any SXM scuba map. With Octopus I've swum with turtles, watched a nurse shark and a huge puffer fish hide out in a little cave together, discovered a troop of GIGANTIC lobster like space monsters patrolling under a wreck, seen an octopus produce an egg sac on a night dive. And Sally, one of the owners, is a fantastic underwater photographer, she takes pics while you dive and you can get them right away on a disc or download when you get back to the shop.
Being in Grand Case is nice too-if you've never been there before, it's a picturesque, laid-back little European fishing town. My dive buddy and I usually stay in Grand Case since the dive shop and all the food we like are right there in walking distance.

If you're diving in the afternoon as well as the morning, you can grab a quick bite in between at the lolos delicious (and cheap!) local open-air ribs/seafood/BBQ stands, a very short stroll down from the Octopus dive shop (I recommend ribs and johnny cake). If you're only diving in the morning, you can take full advantage of being in Grand Case ("the culinary capital of the caribbean") and have a lengthy lunch on the water...just walk down the main drag from Octopus and you'll see lots of places. You can even keep your swimsuit on and eat on the beach at Calmos Cafe (fabulous tuna tartare, greek salad, spicy cumin meatballs, lobsters are my recs) or La California (great pizza, mussels, salads, artichokes au gratin, more lobsters) they'll bring your bottle of chilled water or wine to your lounge chair while you look out at that gorgeous turquoise sea. (Always makes me feel like I'm in a Bond movie- to go from amazing diving to gourmet food on the beach.)
Have a great time and enjoy your diving!
-madamjenn