I'm fairly sure the reef is too far out off of Providenciales (Provo) in the T/C for shore snorkeling. I think the closest point is 500 yds. or so and by then it's deep enough to dive. Grace Bay Beach is a nice sandy beach for a considerable way out. So I wouldn't think the snorkeling there is great. Good diving though. I don't know the other islands but I think it's similar on Grand Turk.
I don't know Bimini well enough from shore - I have dove there but off a liveaboard. Don't know Akumal at all.
My choice would be Grand Cayman. For the 2 divers it's also excellent deep wall diving.
For Mom there's easy entry snorkeling off Cemetery Beach, Smith's Cove and Turtle Reef - which has steps for entry into a shallow cove. It's actually a named snorkel site.
Turtle Reef | Snorkeling in West Bay | iDive Cayman Go here and select snorkeling for 9 other site videos.
Dive Sites of the Cayman Islands - iDive For the Cali or Eden Rock the cruisers are a definite factor - there can be as many as 4-5 ships in port and Eden Rock is 500' away.
Turtle Reef is IMO about the best shore dive on Grand Cayman also - Sundivers is on-site for tanks. Smith's Cove is also a shallow dive a short swim out. Someone recently posted the cruise excursions have found Smith's also. There's a Cayman cruise schedule online to find out the best days to go there.
One other of interest is the Kittiwake wreck - it was purposely sunk shallow enough to be a dual dive/snorkel site. It does require a boat ride out.
You can snorkel overhead and see the main decks and topography of the ship plus take a look down the smoke stack that opens up straight down to the bottom of the hull and the engine rooms.
Kittiwake Sites in Cayman, Kittiwake Wreck Dive - Kittiwake Cayman
Plus besides the Stingray City Sandbar snorkel (the cruise excursion) you can also snorkel above the Stingray City
dive and swim down to participate - it's only 14' deep. Only via boat though. Many of the dive boats offer snorkeling ride-alongs also - on ours the parents dove while the kids snorkeled down.
Testudo - a member here - also publishes a site with a lot more options:
TESTUDO'S SNORKEL GUIDE Cayman Off the Beaten Path