flying or driving?
Compared to Cayman another equally pricey option is the BVI's - there are
day trips to the Anegada marine reserve and many of the smaller islands just for snorkeling. We dove among some of the islands south of Virgin Gorda - at many there were snorkelers closer in snorkeling in the shallows. At places like Little Dix Bay or Mango Bay Resort that's all they allow there - the water is almost too shallow for boats. Everytime we took the dive boat into Little Dix - slowly and carefully - Turtles dove for the bottom. The captain said the whole area is like 10-20' deep. It's their private bay and expensive though - makes Cayman look reasonable.
Typically you fly into St. Thomas and take the fast ferries to the BVI's - takes about an hour. The ferry port is 5mins. from the airport.
There's also snorkeling at the
world famous Baths on the south end of Virgin Gorda. Basically the whole island is Cayman expensive also.
There's decent shallow snorkeling off St. Thomas's East End. Sapphire Beach is one area where a lot of people - including cruise tours - snorkel. Coki Beach is another. There is some recreational drug use on Coki Beach but it's not overt. They also do Snuba there - tethered, surface supplied shallow diving - off the beach. You wear one of those bubble helmets. IDK what the age requirement is.
It's a 1/2 hr. car ferry ride to St. John where the snorkeling is so good they have snorkel trails at places like Trunk Bay.
St John Snorkeling Caneel Bay is also supposed to have excellent snorkeling.
Parts of St. Croix also but IDK where - from there you can also day snorkel trip to Buck Island National Park - it's mostly all that's allowed there - limited diving.
There's decent snorkeling off Nassau also. Stuart Coves dedicates a boat just for snorkelers and they dive shallow reefs. Some are 6-10' from the surface. Coves also has those SUB vehicles - ride on bubble helmeted vehicles that you can ride near the bottom - they're tethered to the boat. But you have to be 12 and 5' tall for that. Downside with Coves is that although they offer free pickup, they're a 45min. shuttle ride away.
There's also several guided tours from Paradise Island (Atlantis mega-resort) that go to another nearby island for shallow snorkeling on 1/2 day trips. If you're a guest at Atlantis there's also Dolphin Cay - she might really like that.
Swim with the Dolphins | Paradise Cay Bahamas | Atlantis Paradise Island