Two to consider:
St. John as Kharon mentioned. Some of the resorts have snorkeling directly off their property. Not the best beaches but there generally is one. Caneel Bay is going to price like the Fairmont - there's also a Westin in Cruz Bay. The better snorkeling is elsewhere though.
Best Snorkeling on St John | St John Beach Guide
IDK if it matters but the only way there is a 1/2 hr. ferry ride from St. Thomas. The fast ferries (big boats) run between all the islands in the region and you can even "drive" to St. John from Red Hook on St. Thomas's east end - there's 2 car ferries. Which also makes it possible to daytrip back to St. Thomas and snorkel at Coki Beach, Hull Bay or Magens Bay in that general area. St. Thomas is sort of dreary though - lots of cruise ships and pretty rundown in places so I don't suggest staying there.
Virgin Islands Ferry Schedules - VInow
Another option is Virgin Gorda in the BVI's. You would take the fast ferry from St. Thomas to get there. It only runs specific days so the other option is the fast ferry to Tortola then a local ferry to Virgin Gorda - both run on the hour. You can also fly into VG on a local airline but all are going to be Cessna-sized twin props - the kind where one of you gets to sit next to the pilot.
There are some very specific resorts with good snorkeling just off their beach. The more affordable one is Mango Bay -
Mango Bay Resort
Our long, sandy beach is naturally protected by a coral reef barrier, providing an ideal place for watersports. The reef also gives Mango Bay Resort its great privacy as none of the larger yachts or big charter boats can find their way onto our shores.
Our calm bay is the perfect place for many activities such as snorkeling....Our guests can also enjoy swimming and sunbathing along the tranquil waters of Mahoe Bay beach. Snorkeling equipment, floating mattresses, and kayaks are provided at no extra cost....Guests can visit The Baths, "The Indians" snorkeling site...
The Baths are a world famous snorkel destination. You can either drive over and walk down the path or go over and snorkel off a local boat.
The Baths National Park, Virgin Gorda, BVI | BVI Tourism That photo really doesn't do it justice so here's some others.
The Baths - British Virgin Islands | Trover.
The Indians is off another island about a 20-30min. boat ride south.
The other
pricier option is the Rosewood Little Dix Bay. It also has it's own private beach - in fact they own the whole bay. There is a boat channel bisecting the beach but it's clearly marked and not used except for resort guests arriving on their yacht from Beef Island Harbor on Tortola - or twice we went in there on the DiveBVI boat - both times turtles dove for the bottom even though we were nowhere near them. Much of the snorkeling there is right off their private beach. It's going to probably make the Fairmont seem
cheap by comparison though. Click thru the first 3 images to see the reef snorkeling options there - both times we were there there were people in the water:
http://www.rosewoodhotels.com/en/little-dix-bay-virgin-gorda/gallery
There are a few other resorts on the island but they're pretty isolated - and two of them are also Yacht harbors (Leverick Bay or Bitter End) so there's a lot of surface boat traffic. Both are pretty isolated so you'd have a long drive to "town" for anything. And it's a very long, narrow island. Some general info:
http://www.bvitourism.com/virgin-gorda
---------- Post added December 31st, 2015 at 12:08 PM ----------
One comment I'd add about Cayman. Many of the resorts are on Seven Mile Beach but the reef is too far out to snorkel (or dive) there. The Westin put down a man-made snorkel trail some years ago but IDK what that looks like now. At the far north end of SMB - Cemetery Beach is an easy entry and shallow snorkel. South of Georgetown Smith's Cove is the other. The majority of the other snorkel spots in the West Bay area might have ironshore cliffs so your wife might be a little intimidated from the water looking at the entry/exit. Turtle Reef is one of the best but here's what it looks like:
http://static1.squarespace.com/stat...65_100000481201574_1569184_299485302_n[1].jpg The steps being the only access point. A few of the other sites have docks/ladders also.