Many years ago we stayed at Sandals hotels - La Toc (bay), Royal Bahamian and Emerald Bay (Bahamas).
La Toc - There are three properties there and this is the most southern and now maybe the largest. We went there twice. First on honeymoon and me as a new diver. I wanted to go back as a more experienced diver. They expanded after the first trip and I think it got too big. The beach there is not the best. Halcyon's beach was better. The dive sites to the south around the pitons are better. St. Lucia diving is good but not great. Better for me than Bahamas diving unless I am looking for sharks.
There are a couple of wrecks and the Lesleen M is good. The other is a good size Japanese freighter/dredger.
The diving near Royal Bahamian (New Providence - Nassau) was not that good however the air and water temp in January was 64 F with high winds. However there were quite alot of sharks and our dives were not feeding dives as we took advantage of the staging areas of other dive ops - the sharks know. Lots of sand in general but we did try a wall to the north but the DM had us swimming against the current the whole time. Short dive, even by Sandals standards.
Small property and beach. Not sure I would snorkel off the beach but didn't try.
Emerald Bay, Great Exuma, Bahamas - We stayed here because they had just bought a Four Seasons hotel and were doing a soft opening with free butler service (awesome). The diving here was quite alot of sand and not that interesting. We were scheduled to do a wall one day but a lightning storm cancelled it. We bailed on Sandals diving and went with a local dive op. This hotel has a fantastic large beach and the property was good. Lots of sand and we could wade out 100 yards to chest depth. Maybe not so good for snorkeling. We did dive a small blue hole which I had not done before. There was a small wreck but don't know it's condition now (been 13 years).
Of the three, I would say St. Lucia was better diving and topside is much better (pitons and more mountainous). I just can't dive with Sandals anymore as it's too restrictive with short follow the leader dives. I know it's free but so does everyone else including the honeymooners, which I was one the first time, many many years ago. These were my first ocean dives.
If you want wrecks, and of the three, St. Lucia is better. We never tried night dives at any of the three locations. I would say that if you want an outside operator, you will have more options on New Providence (Nassau) but the diving is not as good but does offer dedicated shark dives. Great Exuma is small and has less dive ops. St. Lucia has other dives ops that have good reputations.
For any Sandals location you choose, I would send emails to the outside ops beforehand and see what they say.
If I had to choose between the three I mention here, I would choose St. Lucia. When we were there, Halcyon was buggy as hell but could have been a specific time of year (August and believe me it was hot). Btw, at the time and maybe still active, you were able to go to the other properties for play or dinner but it had to be organized in advance.
I am judging diving as compared to FP, Fiji and Cozumel.