I think that I can help you since I have been going there for many years - every year around early December. (December 6-13 this year)
I do not know what you consider good snorkeling, but if I had to rate St. Maarten or St. Martin snorkeling, I would give it a 3 out of 10 - where 10 is Heaven.
But since you are going there no matter what, here are the places that I like to snorkel at. Almost all of them require a very long swim, since most of the snorkeling areas that are close to shore are pretty much dead. Plus, I do not know how good a swimmer/snorkeler you are, and you need to judge the sea conditions (waves, wind, and water clarity – bluer is better, greener is no good) before considering these sites.
1. Marcel Beach (northeast) - if you go to the mouth of the bay, either side of the bay is decent, and going around the little islet that is outside of the bay on the left is pretty good. But remember, this is a long swim.
2. Orient Bay (east-northeast): if you go in near the BBQ shack that separates the nudist resort from the rest of the bay and swim to the right, parallel to shore, you will find turtle grass and small patches of rock and sand (coral is dead), quite a few fish, small lobsters, and sometimes an eel. Then when I get to the end, I take a left turn and continue looking at the small patches of rock. Sometimes you might see squid and a few more small lobsters. Then comes the tough part, which is to swim against the waves quite a long way to go to Green Cay, across the bay. If you are lucky, you might be rewarded with a stingray or maybe even an eagle ray, but this is an area that is also risky because there are jetskis and windsurfers - so you always have to be alert. Once you get to the sandy (left side) of Green Cay, you can continue snorkeling up along the left side of Green Cay. This side is sheltered from the wind and waves so it will not be hard. Initially you will find a lot of dead coral, but the further up you go, the better the condition of the corals. If you do not choose to swim across, you can ask a water taxi to take you across.
3. Coconut Grove or Baie De L'Embouchure (east): this is also a very long swim, with some areas being very shallow and covered with turtle grass, and usually there are quite a few waves once you get outside the bay, but there are areas where the coral is in decent shape, and the fish and small lobsters are nice here too. I have even seen a nurse shark here but that is entirely luck, since it was once in 18 years.
4. Dawn Beach and also Baie Lucas (east): the corals here are not bad and the reef is very close to shore; however, these bays face directly east, which is where the wind comes from, so these two, which are next to each other on the east coast, are usually quite rough with very choppy conditions.
5. Baie Rouge (northwest): there is decent snorkeling going to the right towards the arch, and on the other side of the arch. Again, not necessarily a short swim. I believe this arch is called Le Trou du Diable. The sand on this beach is magical: one stomp of your foot on the ground and the sand vanishes from your foot.
6. Mullet Bay (southwest): the snorkeling here is fairly easy along the rocks on the left side of the bay and around the corner of the bay. Sometimes there are quite a few fish. Be careful with the overhanging rocks since you can easily hit your head against them.
7. Pinel Island (northeast): if you drive to the Cul-de-Sac, you can take a water taxi across to Pinel Island, where once gain, there is some decent snorkeling the further away you get from the sandy beach area, since most of the coral near the beach is dead.
There are many other beaches on St. Maarten (the claim is 36 beaches) and some of them such as Plum and Friar's are reputed to be very good for snorkeling but I have never found the snorkeling there to be interesting.
To go to Anguilla, you need to go to Marigot and ask for the ferry to Anguilla. It is about a 15-20 minute ride in a fast boat. It used to cost about $20 roundtrip but probably costs a lot more now. You need to bring a passport since you will go thru Immigration there and they will go thru your belongings for contraband materials.
Once in Anguilla, it is best to rent a car, get a map and go on your own to the places that you wish to go. I recommend Shoal Bay East, which has a nice coral reef within easy access to shore. In my opinion, the most beautiful beach for snorkeling in Anguilla is Little Bay, but it is very difficult to get to - it might be better to hire a water taxi to take you there. Lastly, you can take a water taxi to Sandy Island, which is very pretty and also has some decent snorkeling all around. I do not remember the name of the bay where you can find the water taxis but I am sure that you can find out by asking once you get there.
All of the above are on the north coast of Anguilla.
