Cayman Islands or Turks & Caicos are options. Both are fairly pricey if that's a concern.
Grand Cayman has dive sites that are also recognized snorkel sites. And 365+ marked dive sites. Stingray City for rays - either dive or snorkel. We've seen dozens of Eagle Rays on the North Wall/NW Point area also. Usually in packs of 12-15 flying over. And Turtles are their national symbol. They get out from the Turtle Farm frequently and return to the reef nearby. There's also sharks at certain dive sites.
iDive | A Social Community Designed for Divers lists both snorkel and dive sites - most with video. Smiths Cove or Cemetery Beach is supposed to be a better snorkel than dive. You can also snorkel or dive the Kittiwake, it was put down as a dive site. The top almost breaks the surface and the bottom is around 60'. Google Testudo's Guide, he lists Grand Cayman snorkel sites with pictures.
Little Cayman has Bloody Bay Wall offshore. It's one of the top wall dives in the world. It's deeper snorkeling there though.
Bountiful Bloody Bay Wall | Sport Diver I've never been there but it's on the list. There's supposed to be big Grouper there since they're protected.
Off Turks and Caicos we saw sharks on just about every dive in nearby West Caicos or French Cay. At one site in French Cay I stopped counting Stingrays when I reached two dozen. Most resting on the bottom. Saw several big grouper and frequently turtles. Also a few Eagle Rays. Both locations are a longer boat ride from Provo - the main island - or Grand Turk, a smaller island which is also the cruise port. In the winter months, the Humpbacks go near Grand Turk on their way to the Silver Banks - snorkel only. I don't know about snorkeling on either island but have seen it mentioned.
If your focus is more on snorkeling, maybe the Virgin Islands also. St. Thomas has both off the East End, St. John has Trunk Bay and other snorkel trails plus 1/2 the island is a Marine Reserve. St. Croix also has some good snorkeling, the Frederickstad Pier or Buck Island National Monument just a short boat ride away.
We spent a week on Virgin Gorda a few years ago. The Baths there is world-class snorkeling. People come over from Tortola, St. John and occasionally St. Thomas just for it. There's also shallow snorkel spots at some of the smaller islands. At several dive sites we saw snorkel boats also.
A couple of the resorts also have private bays where they only allow snorkeling - the coral comes right up to the surface so boats can't get in there. But they're pricey. (Little Dix Bay or Mango Bay Resort)
Good diving also, we frequently saw turtles, an occasional shark. Quite a few stingrays. It's shallower diving with fans/soft corals. I'd rate the diving not as good compared to the Caymans/T&C.
