Nice thread comparing Caribbean dive locations shaping up here.
The OP has an impressive list, one needs to consider seasonality of locations for both diving conditions and sea life.
For example Turks and Cacaos can provide outstanding diving with frequent Caribbean Reef Shark encounters. Hunting Black Jacks and Nurse sharks make for some exciting night dives. Best done via a liveaboard as Provo is expensive and boring IMHO. I might have to add Grand Turk to my list of places to go for land based diving?
Belize is a big place with big critters. Spotted Eagle Ray and Nurse shark are common throughout. I have seen Black Tip, Hammerhead and Bull Sharks in Belikan waters. Placencia has a good shot at Whale Sharks in season. The outer atolls provide some healthy reefs with schooling Permit and Horseye jacks. I would consider doing a Liveaboard in Belize next time as the best diving seems to be off shore.
Roatan has dramatic walls with excellent topography, several good wrecks, shore and boat diving. Big Black Grouper and the best shark feed at Cara a Cara for big stuff. An abundance of Sea Horses and Macro. Backpacker West End or All Inclusives provide lots of options.
Cozumel is a dive circus, superman drifts, viva Mexico. Lots of variety from Nurse Sharks to the Splendid Toadfish. Shore diving is doable but limited by current. Thy have some of the best priced package dive deals in the region, my only problem is I have yet to find a dive op there that I would recommend. The Playa del Carmen Cenotes / Cozumel drifts combo is hard to beat.
Jamaica is rough and tough, but has the richest culture I have fond in the Caribbean. The Food, Music etc. are hard to beat. Their reefs tend to be in bad shape but for big Southern Stingrays I can't think of a better place. Sun Divers Negril is an excellent shop, I did most of my early dives with them and hear they are still a great outfit. Has anyone dived Port Royal off Kingston?
Saba was a bit of a disappointment but I had very high expectations. Strong currents kept us off the premier dive sites most of the time. We did see Manta Ray and Caribbean Reef Shark as well as Sea Horse and various Nudibranchs. There are a few spectacular dive sites there as well. No shore diving or beaches and pricey, yet the mountain in the sea landscape is unique.
Little Cayman, shhhhhh. The bloody bay wall is spectacular with roaming Reef Sharks. Limited topside options but the diving is some of the best in the Caribbean.
I just got back from Curacao, doing the drive and dive deal. I can see the appeal of a shore diving destination and will have to do Bonaire soon as it makes #1 on so many lists. Not sure how it would be for beginners though. Curacao has some truly unique coral formations, and and abundance of Eels. Sharp tailed, Spotted Snake, Golden spotted, big Greens, Spotted, Goldentail eels. If you are into Eels this has to be it.
What about the Florida Keys? They have a nice assortment of wrecks as well as Tarpon and Goliath Grouper. The reef structure here is not as spectacular as other Caribbean locations but for many of us you can drive there with a ton of gear. Good shore and boat dives, might be a great place for beginners?
Not technically Caribbean but Bahamas has great visibility and light with white sand bottoms. The big Loggerhead Turtles there are a hoot. Lots of options in Bahamas from the backpacker style Blackbeards liveaboard, land based resorts in either Nassau or Freeport, to tons of little getaways throughout. My buds rave about Cub Med, San Salvador for Hammerheads.
Nice to see Dominica at #2 on the OP's list as this has been on my must do list for a while.
Providencia via Costa Rica just went way up on my list, any big stuff there?
What about Corn Islands, Nicaragua?
Some have been saying that the Caribbean is lacking as a tropical dive destination on this thread. It is easy for North American divers to get a little jaded regarding our local tropical waters. The reefs are in poor shape with a lack of abundant fish as compared to some places in the Indo-Pacific region but don't forget we have the coolest Sponges on earth often with ideal conditions and visibility. Topside the Caribbean is hard to beat, look who invented the BBQ and the hammock.