I went diving with Ocean Explorers for a week in 2013 and was very pleased with their operation. Although I have been back for a day by cruise ship, I haven't been diving there since then, so I can't speak to the quality of the diving these days. Located at Simpson Bay, OE is an excellent dive op and they have consistent positive reviews on Trip Advisor. They are one of the few privileged dive operators licensed to do a dedicated shark dive. Back then, I didn't have a camera, and they had a photographer on staff that would take photos for you and then you can purchase the package. Very reasonably priced for a ton of great photos.
I stayed at the Atrium Beach Resort, which was a very short beach walk to OE. Decent condo-style resort with a kitchen, air conditioning, and security on the property, which is important on St. Maarten wherever you stay.
A trip over to Maho Beach provides great plane spotting, watching them come and go. The attached photo is not altered. This is how close the planes come in. It's a fun day at the beach!
Couple of travel tips:
Don't drink the tap water, no matter who tells you it's safe. It is not!
If you rent a car, be ready for an adventure in driving. Since there are no sidewalks on most streets, you share the road with cars, bicycles, motorcycles, and pedestrians. It's a bob-and-weave situation. Lots of horn honking. It's not the aggressive honking we do in the U.S. but a short beep-beep to let whatever moving obstacle is in your path know that you're there. I didn't rent a car except for one day when I took a non-diving day trip to the French side, which was very nice. The rental cars are in really poor condition with a lot of body damage, so be sure to take pictures inside and out of the whole car before you leave the lot.
I know this is more than you asked, so excuse my long-winded post. Have a great time!