We've been to Grenada and Bequia, and dove Carriacou from a liveaboard. Dominica is still on our list so no info there. I also love snorkeling but can't comment as I don't take it into account when planning a trip and don't usually remember - it's more like if a good opportunity happens to present itself, I hop in.
While we didn't do a lot of dives on Grenada, I wasn't overly impressed with what I saw. Part of the problem was viz sucked, and I don't know how typical that was. It seems other people have had better dive experiences there. I thought it was a great island topside though. Interesting place to explore, lots to see. It's bigger and more populous than the other places with a real (and interesting) city, so a different experience. By far the easiest of those to get to. I'm not sure if I would go there specifically for a whole dive trip, but I would happily combine some days there with another place if it made sense travel-wise.
Bequia is a very cool place topside. Lots of interesting stuff to explore given the small size. The diving was pretty good, and there were some unique dives. We were also able to dive Isle a Quatre by day boat from there. Surprising number of pretty good restaurants for a small out of the way place, probably because it's a big stop for the sailing crowd. We combined it with a trip to St. Vincent (one of our favorites), and took the ferry between. I would definitely like to get back to Bequia. (We've been going other places, holding out for them to finish the new St. Vincent airport and maybe get some real airlines and planes flying there.)
I don't remember the diving on Carriacou in particular without digging out an old dive log. My memory says fine but nothing to make me go to the *trouble of getting there to stay. If I wanted to go to a small unpopulated island to dive, I'd probably sooner return someplace like Little Cayman. But I wasn't actually staying on land, so for all I know it might be similar to Bequia and very cool. (Actually, I just googled "getting to Carriacou" out of curiousity and supposedly there's a high speed catamaran between there and Grenada. That's something I may look into...)
*(I define "trouble" as anything that involves more than one puddle jumper, planes so small taking my dive gear might be a headache, or having to fly LIAT.)