I've heard Belize is the best dive spot in the world, so I would like some feedback on that.
It is certainly in the top 5 Caribbean destinations for diving. Remember that Belize is a country made up of the mainland where cruise ships dock- and also of the many barrier "cays" lying well offshore to the east. That's where the diving of note is- some considerable distance from your cruise ship. For this reason- absolutely go with the cruise ship's program. They had better be keeping you moving from the minute you disembark!
If you had to choose between Belize, Roatan or Grand Cayman for the diving... which one would you dive at?
This largely depends upon your needs and abilities. You can't beat the experience of StingRay City in Cayman. Deeper- there is Tarpon Alley. Right near the docks- the Labrynth of Eden Rocks. Quite barren but fun.
Roatan is best done a week-at-a-time, you have to get hooked up with a good DM to see the "small stuff"... that which Roatan is best known for. It takes a few days to get your buoyancy under control to see such macro critters. Most come to Roatan and are disappointed because they can't see any "big fish". This is quite true, but then again, either their DM's are inept of the divers themselves are incapable of seeing the pointed-out Seahorses and Pipefish and tiny colorful cool stuff that this diving is all about.
Most Roatan cruise ship visitors get shuttled over to Anthony's Key Resort for the Dolphin Show. There are two day-dive ops worthy of note: Native Sons and my far and away choice- Subway. They will meet and move you to their ops.
Other AI's can and will arrange visits, but this would be desirable if you were thinking of spending a week "next time".
And has anyone come up from under the sea long enough to do a Canopy Tour, and what did you think of it?