Here's advice:
Bring sunscreen. OK that one is a no brainer.
If I recall, Sept - Oct is a big 'holiday' for the local citizens. Things shut down for maybe a month. Stores, restaurants, everything so check before you go. Casinos are on the Dutch side only. They are not Las Vegas. You do not need your passport to move between Dutch - French SXM. Check the cruise ship schedules - do not go into Phillipsburgh days ships are in port. Phillipsburgh is jewelry and shopping mecca - even things for divers like watches and UW camera. Know what you are looking for before you get there. Picked up Citizen Hyperaqualand at a sweet discount. Negotiate. For foodies - do plan a couple good dinners out - try Gran Case on the French side. Local eats - try Talk of the Town lolo for great chicken and ribs, also in Gran Case. Or hit the roadside grills. Go to the French Grocery store (I think it's Gran Marche) for wine and cheese and snacks. When you are not diving, hit the beaches. SXM has a different beach for every day of the month. Bring your fins and mask to the beach. Marigot has a pretty cool Wednesday market day - local fare, as well as a chance to practice your French, shop, eat yummy pasteries and coffee, see where Speed II (the movie) was made. Check out the marinas - Simpson Bay Lagoon often holds some of the worlds mega-yachts. Do a drive around the island. Orient Bay has a CO (clothing optional) section. Don't stare or take pictures, that's just rude. Do check out the beach, it is gorgeous. The butterfly farm can be a good way to spend some time. There can be traffic jams, so if you have to be somewhere, allow enough travel time. Diving is good. I saw my first shark there so it will always be near and dear to my heart! Op will pick the site based on conditions. Most are hard bottomed, patch coral/seagrass- no walls or thousand foot abyss. Beach diving is not common (we did it once as a check out dive and had a hard time finding someone willing to rent tanks). No big named wrecks. Normal assortment of fish, turtle, marine mammals, cartilage types (rays, sharks, squid, cuttlefish) and octopi. Easy peasy diving - roll or stride off the boat. (Saba is deeper, larger things - grouper, open water fish. No beaches on Saba, no run off.) Don't buy a timeshare. Lastly, not to be an alarmist, be safe. There are certain neighborhoods you don't want to be in or drive through after dark. There have been robberies, cars forced off roads, breakins, beatings. Leave nothing of value in your car. Leave the windows cracked and unlocked at the beaches. Keep wallets and purses close. Be wary. The daily paper can be found here:
The Daily Herald -. PM me if you have any specific questions I haven't addressed.