I don't think that there's anything that I would consider "great" in St. Martin.
The following are places that I go to - they are not secret, but they are far enough from shore and thus require enough swimming to discourage many from going that far.
1. Outside Marcel Bay on either side and specially around the rock off of the left side.
Even going across from one side to the other, outside the bay (mostly sand with turtle grass) gives you the opportunity to see stingrays and huge conchs, and if you're really lucky a flying gurnard. I've never ever seen another snorkeler out there.
2. Orient Bay - going across to Green Cay (careful with windsurfers and jetskis) and up the west side of Green Cay. Occasional sightings of Eagle Rays in the middle of the crossing, as well as stingrays. The area on Green Cay closest to the small sandy beach is dreadful - just too many people so it's all beaten up. The further you go, the better it gets because less people go up that far.
Even the shallows at Orient might surprise you sometimes - not for lush and healthy corals but for the life that you can find. Go in just to the right of the restaurant that separates the nudist resort from the rest of the beach. The patch of rock that runs almost parallel to shore has quite a few fish, and baby lobsters. Occasionally, I've seen scorpionfish here. If you continue going parallel to shore heading to the right all the way to the end, you will occasionally encounter patches of rocks here and there - you can always count on finding baby lobsters here too, as well as baby Highhats. On occasions, I have seen squid, morays and barracudas.
3. Baie de L'Embouchure, also known as Coconut Grove - the beach immediately south of Orient. Here again, the further out you go, the better it gets. Try going from the point on the left - it will save you from having to swim thru the shallows. The closer you get to the breakers, the better the coral. Do be on the lookout for windsurfers and kitesurfers. I've seen lobsters, school of Blue Tangs, and school of Snappers (Schoolmasters) here, and one Nurse shark once.
4. Dawn Beach - it used to be good, especially on the outside of the reef as well as just inside. The shallows are quite beaten up. Now that they reopened a resort there, who knows. Plus, this is a tough beach to swim in because the wind is always coming in from the East.
5. Red Beach or Baie Rouge - the best part is going to the right, on the other side of the arch, what they call the Devil's Hole. Juvenile Queen and French Angels can usually be seen here.
