I spent seven years running charters in the BVI's and have run down island a dozen times. I believe the BVI's to be pretty darned good snorkeling while the Grenadines are not. You have to work hard to find good stuff there whereas in the VI's good snorkeling can be found all over the place and the ease of access is great. BTW...your list of good BVI sites misses many great snorkels such as the Indians, Mountain Point, Cow Wreck, etc. Have had many guests say the snorkels I took them to were superior to Belize and Cancun.