I am going to respectfully disagree with lucianmermaid just a tad. I completely agree with her comments on Salt Pond. We always saw a lot of creatures every time we snorkeled that spot and Blue Cobblestone, just around the corner east of Salt Pond, is one of my favorite snorkel spots on St John. It gets rather deep out there and you get to see larger pelagics and a resident shark as well.
Long Bay south back towards Nail Bay on Virgin Gorda has outstanding snorkeling, as does Mountain Trunk. The double reef at Mahoe on VG is massive and measures up well versus shore access spots on St John.
I agree that Jost is better visited from St John rather than VG...it is considerably closer. However, the Indians and Norman, though closer to St John, are easier to access from VG. On St John you have to go around the length of the island and then over to West End, Tortola to go through customs and then back over to the Indians and Norman. This takes about an hour or so. From Yacht Harbour on VG, it only takes 40 minutes. Since you are already in the BVI there is no need stop for customs. Either way, the Indians are an excellent snorkel spot that you should not miss.
Edited: 9:21 am, May 28, 2013