Firebeef got to all my recommendations first. The only place I could add is Grand Turk in the Turks and Caicos. Ex-Brit island with really good wall diving, lots of marine life, spectacular night diving if you can time your trip over a new moon. Try doing a no-lights night dive on the north end at slack tide, there's so much bioluminescence it's a religious experience. Seriously. It's dive crack. Little or no current, reefs start 5 mins offshore at about 30 feet. It's a great place to get your first dives under your belt -- I brought my daughter there after certification and she had a great time. There were Navy divers there at the same time doing crazy stuff, too, and they also had a great time.
If you go to Roatan, bring lots of bug spray. The diving is great, but the no-seeums are vicious.
Hijack alert:
I was going to put this in 'Be Prepared' because people on dive trips laugh at my first aid kit: "Whatcha need sutures for?"
That is my dive buddy Ben below. He's a young Brit expat I buddied up with on a liveaboard. Good diver. We were hunkered down back-to-back at a bout 30 feet, eyeballing the sharks when one darted in and hit his hand. We speculated it was his watch that brought it in.
Check out the video below: "One of these sharks nibbled on Ben." Just prior to taking this video a hammerhead had demolished a Southern Stingray in three bites. The commotion and blood brought in a horde of Caribbean reef sharks and a couple of bull sharks, who proceeded to demolish a large grouper that was in the area. They are just finishing up at the start of the video, you can see them off in the distance. We thought everything had calmed down when a medium reef came in over my shoulder and hit his hand from behind. "'At 'anker 'it 'e." (That wanker bit me!) He bled like a pig from such a small gash.
Crappy video, but good considering I used a dinky Canon Digital Elph. I take snapshots, not photos.
[Shark pass]
[One of these sharks nibbled on Ben]
Do not stick your hand in a shark's mouth.
Ben_shark_nibble2.jpg
Half-way done. No one ever gets to laugh at mugwump's first aid bag ever again.