It depends on what type of diving you want to do and what you want to see. Grand Cayman & Belize have a good variety of sites.

We've gone to Grand Cayman for many years (8 or 9). My uncle lived there so we went for a couple months a year. The diving is phenomonal for fish but as for coral not the greatest. The cartography is unlike anywhere else in the world and they have some super sweet wrecks to dive(USS KITTIWAKE). Although corals aren't that great, the Caribbean is not known for corals and no matter where you go the ocean is not going to look like some peoples fish tanks. It would help to know if your going to be diving or snorkelling.

If you go to Grand Cayman i suggest you also take a little trip to Little Cayman or Cayman Brac(Sister islands of Grand Cayman Cayman and much smaller). The diving is equivalent maybe even a little better. Little Cayman is known for its cool dive sites. When we went there was a site where there were 2 Tagged Nassau groupers that you could cuddle with and pet. The fish would even snuggle up to you if you didn't continually pet them Lol. (keep in mind these groupers are 3-4 feet long and are extremely endangered. :)) If you go to little cayman get a guide to take you to Tarpon Lake. It is a brackish water lake that was created by a hurricane and tarpon got carried in there during a hurricane. The fishery is crazy, even if you don't fish its a cool thing to see the tarpon school chasing all the fish.
Cayman Brac is more known as a party island. I have never been there so have no personal experiences.

I plan on snorkeling, and diving if time were to allow it. I've never been, though it's something I've always wanted to do.

I do understand that the Caribbean has been on a rapid decline. I've watched numerous documentaries on the subject.

Don't worry about price. I want to see a diverse fish population, and hopefully some decent coral. I would also consider Hawaii. Though, anything further than that is out unfortunately.

From the videos and pictures I've seen, Belize is definitely the way to go. I'll have a look at some of the other suggestions!

seriously... this is one of my top 2 places....

I have been to Hawaii and Belize...

http://www.bahamasunset.com/sunset/index.html

I'm going to seriously consider that, hfp75.. that looks amazing!

We snorkeled in Belize in 2013 off Ambergris Caye. AMAZING - but it was my first time ever swimming in the ocean, so I have nothing to compare to, but we saw fish, turtles, sharks, sting rays and off the boat dock, a sea horse. Loads of life in the water there. Tons of coral both close to the surface and deep. You could actually just take a paddle boat to the reefs off the beach. Saw others doing this.

I'm heading to Mexico in February, will take my snorkel gear again (not sure on actually diving)....but that's just me.

Unless you have an unlimited budget price does matter. Prices vary from country to country in the Caribbean. Both for resorts and for overall cost. You can do cheap like Cuba or DR or hit the more expensive places such Turks or St Vincent.

Another thing is that some places don't have great snorkelling from resorts (although beaches are fantastic) so you have to do excursions to get in the water. Keep that in mind.

Yes, of course they vary. I am aware of these circumstances. I have a very open budget, and am willing to pay extra for the underwater sights. If the better snorkeling is in a more expensive country, then so be it! I'm going to see the fish, not the resort. ;)

Go slightly east. Bali :)

The best diving in the Caribbean is subjective, but most of the diving community (from what I have seen/heard/read) agrees that the Bay Islands are easily among the best. Roatan and Utila are renowned for a wide variety of life, from whale sharks to lobsters to all kinds of reef fish large and small. The reef is healthy and vibrant, no signs of decline from what I have seen myself. Easy access to beach snorkeling in lagoons and the inner reef, fantastic reef walls on the outer, and beautiful warm water year round. Plus once you get there it's pretty cheap, the only problem is getting there... three or four flights, an overnight in a central american airport, and then either a rough ferry ride or a little plane over the water. I think there used to be direct flights from Houston but I couldn't find any last time I looked.