Hi i_wanna_be_aquaman,
As you know it is not a dedicated dive destination, nor IMHO even an especially attractive tropical island overall since it has been so aggressively developed.
In my experience, best bets in diving are Trade Winds Dive Center, located in Great Bay Marina, or Ocean Explorers. Tom Burnett is the owner of TW,and Leroy & Dominique of OE. The latter a coupe of real Kool dudes. Simpson Bay diving takes place on some small & rather ordinary reefs a couple of miles out.
If you are a serious diver, Saba is the place to go (can do a day trip). About 30 miles south of SM. World famous diving. Take Winair in from St. Maarten (the landing is worth the price of a ticket) or The Edge or Voyager ferry. Three dive shops on island, all quite good, all dive same sites. Personal favorite is Saba Reef Divers (SRD). Tent Reef is are excellent. If advanced diver, the pinnacles, especially Eye of the Needle, are a must. Just below recreational limits is the "Cathedral." Diving more high voltage than Dominica. Once you've climbed Mt. Scenery, there is nothing to do. No beaches, no shopping.
If committed to SM, take the brief ferry ride to Anguilla, off the southwest shore. It is a relatively undeveloped island with a coupe of real upscale resorts good for lunch or a drink after diving. Some reefs & a slew of relatively recently sunken wrecks off northern point. The M.V. Oosterdiep is pretty cool. There used to be a good dive shop over in Sandy Ground, called the The Dive Store. Think it's new management since the hurrricane damaged it.
Have fun.
DocVikingo