I have been to all the mentioned locations. The snorkeling on SXM is not in the same league but there are places that are acceptable for snorkeling. Shoal Bay on Auguilla is much better. I placed on youtube a few clips (video and pictures taken by me) showing snorkeling near Green Kay off Orient Beach and a youtube showing various beaches on SXM at
https://www.youtube.com/watch?v=Ty0EPfWeZRE&sns=em

https://www.youtube.com/watch?v=zi_dgvO0tec&sns=em

You will see on the second clips still pictures of marine life taken by me just off shore Orient Beach. Every day, I would see something different.