Shore diving or boat diving for you? Remote or near stuff? Looking for more of a budget place, luxury or someplace in between? And what else is important, good food, shopping, other activities, or just quiet?
I haven't been in years but I thought the snorkeling within the lagoon off the Reef Resort (East End Cayman) was pretty nice, not so much a reef just scattered coral, but saw turtles and rays and lots of things in the seagrass, and there's other things out by the rocks as long as you're careful. And their whole beachfront/pool area is pretty nice. Also in front of Morritt's next door especially around the dock, though I think that got rebuilt after a hurricane some years ago and there's been lots of other construction there, so I don't know what it's like now. That's the quiet end of the island, less restaurants and stuff and you'll want a car, but not bad to drive to other parts of the island to do some stuff too. And they could go do Stingray city or a day at Rum Point. (Best to pick non-cruise ship and maybe non-weekend times for that.)
Kura Hulanda Lodge on Curacao is sort of remote but it's nice and there's good shore diving/snorkeling, and dive boats leaving from the dock. If they want to mix it up, also lots of little beaches where they could snorkel while you shore dive, some with facilities like bathrooms, snack bars, and dive shops.
I think a fair number of places on Roatan would work, though I'm not personally familiar with many of the places. Google snorkeling roatan for a start, or someone may be along to suggest specific places. Maybe Cocoview would work, though that is very much a divers place with no beach and a bit rustic if that matters (and I wasn't a fan of the food.)