Some options where you'll find everything on your list...
I disagree with Bonaire as a suggestion because it doesn't meet the OP's wish list. I don't believe there are any direct flights from Boston, there are not many activities for a non-diver, and theft is a significant and persistent problem. There are lot of things I love about Bonaire, but we are divers (and nerds!) and so it is a pleasant option for us, I don't know about a non-diver. Not many beaches on Bonaire but there are some. There is very good diving but not many sharks, you might see a nurse shark. There is very good snorkeling available from shore.
Curacao is more developed near the city and there are more non-diving things to do. I don't expect that there are any direct flights from Boston. I haven't been there in awhile but my understanding is that there are also some theft problems, although maybe not as commonplace as Bonaire. And like Bonaire the diving and snorkeling are lovely but not many sharks. There are more beaches on Curacao than Bonaire.
On Provo in the T&Cs there are lots of chances of seeing sharks, but I don't think you can fly direct from Boston and as far as I know there are no great shore diving or "snorkeling from shore" options. Grace Bay is a fabulous beach and there are activities likely to be of interest to a non-diver. Just an FYI, you should drink bottled water in the T&Cs because they have had outbreaks of Norovirus. Also be aware that the best reefs are a long ride from shore. You may spend much of the day in boats and that could be boring for your non-diving spouse.
I think that Grand Cayman comes nearest to your wish list. Jet Blue has direct flights from Boston to GCM. There are a lot of shore diving and also snorkeling options from shore. There are onsite dive ops at many of the most popular shore dive sites like Turtle Reef and snorkeling is available at the same locations - although it may be difficult to find an "instant buddy". There are lots of great beaches on the island and Seven Mile Beach is in the same class as Grace Bay.
You are likely to see some sharks in GC, mostly nurse and reef sharks but not as many as the T&Cs. You will see lots of turtles and rays and groupers though. There are many nearby attractions and cultural destinations as well as many shopping and restaurant options.
All of these destinations are expensive with the T&Cs and GC being the most expensive, but there is a wide range of budget options depending on venue and season. Pick out the place you want to go and and when and we can help you find accommodations and a dive op to fit your interests and budget.
If you decide to travel during tropical storm season (June through November) you are more likely to encounter bad weather in the Caymans or the T&Cs that in Bonaire or Curacao but there are no weather guarantees wherever you go. The water temps are cooler in the Caymans and T&Cs in the winter and early Spring as compared to Bonaire and Curacao.
On all of these islands I would suggest renting a condo or an apartment with a full kitchen to prepare some simple meals and still have options for going out for a nice dinner meal. An apartment or condo is usually quieter than a hotel or resort so I don't think you will find loads of children. Some larger resorts will offer separate pools and facilities for adults only.
I also recommend renting a car on all of these islands. You definitely need a car on Bonaire and Curacao, you can probably avoid renting a car on T&Cs and GC if you stay in the center of Grace Bay or on Seven Mile Beach, but you will miss a lot, and cab rides get quite expensive. And if you stay on the sandy beach you usually will not find good snorkeling or shore diving.
You can find modern, clean, bug-free accommodations on all of these islands but mosquitoes are an ever-present issue in the tropics. I am very sensitive to mosquito bites and I think that the Caymans have the best mosquito-abatement programs, but you can still get bitten, use protection.
I have never been bothered on the beach by vendors in any of these destinations unlike Jamaica and Cozumel. And I am sure that you can find options for yoga and tennis on all of these islands.
If you make arrangements with a dive op on any of the islands they will help you find a buddy for the boat dives.
There are places in the Bahamas, like Nassau, where your can find sharks along with many options for non-divers. I think that the diving is better in the 4 islands mentioned above by DiverSteve but I am sure you would still have a good vacation and you would be able to find direct flights to Nassau.
There is also great diving available in southern Florida and the Keys, and plenty of things of interest to a non-diver. You could probably find a lot of direct flights to Miami or Ft. Lauderdale and rent a car. Lots of fish life and wrecks but the reefs are not quite as nice as the Caribbean IMO and there may be longish boat rides.
St. Croix is another good Caribbean destination with options for both divers and non-divers. Probably no direct flights from Boston. Not many sharks but the Fredricksted pier is a world-class shore dive and the snorkeling there is also great.
Here's an off the wall suggestion because it is a long trip and probably not direct, but have you been to Hawaii? Interesting and beautiful diving and lots of things to do. I don't think you are as likely to see sharks but the night diving with Manta Rays off Kona is fabulous.
I think that there are still a lot of places for you to visit. Most of them will not meet all of your criteria but each is wonderful in its own way. The best thing to do is to check them all out and then decide what you like best!