Ditto to the recommendations for Aquamania (Dive Adventures). We've been to St Maarten several times and appreciated Vinny, Mikey and Mano and the rest of the staff. Mikey is a fantastic dive leader. He is so comfortable and familiar with all the dive sites, you'd think he was really a fish disguised as a man.
Our favorite dive sites are the wreck of the Porpoise -- a tug boat sunk as a diver attraction that sits upright and is only about 3 minutes from Simpson Bay. It sits in 100' and has some awesome swim throughs, and loads of fish. The goatfish were so thick the times we dove there, I could hardly see the wreck. We also like Proselyte Reef, Elevator Shaft and Grouper Reef -- all very fishey and fun sites. I found diving there comparable to most reef sites near Tavernier, FL and Marathon, FL.
We've dove there in October and May and viz was 80+' both times. Surface conditions are generally calm, but you may get 2-3' once in a while. We wore 3mil suits and were comfortable.
Ditto also on taking the EDGE to Saba for a day. Man O' War Shoals is a not-to-be-missed dive site: two sea mounts that come within about 25' of the surface. Max depth ~75'. The current can be a bit of a challenge. If you encounter this, just tuck into the lee of the sea mount and enjoy the incredible variety of healthy coral, gorgonians, turtles, and more fish that you can shake a lobstertail at.