The Blue Hole is probably the most famous dive spot in Belize. It is a wonder of nature that reaches depths of over 400 feet. The Hole is enclosed by an almost perfect circle of coral, and divers go as deep as 150 feet to see the stalagtite formations in the caverns below the surface.
The boat ride to the Blue Hole from Ambergris Caye is about 2 hours long and leaves at 6am. The ride can be rough, so I recommend taking a motion sickness pill -- even if you don't usually need to.
You will be fed breakfast and lunch by the boat staff (we went with Amigos Del Mar). We stopped for lunch at Half Moon Caye, where you can see red-footed booby birds as you walk along conche shell-lined paths.
We were taken on two other gorgeous dives after the Blue Hole off of Half Moon Caye. They are drift dives, which means that the boat picks you up wherever you surface. We saw turtles, eagle rays, toadfish, gorgonian trees, and lots of other great stuff.
The cost for the whole day is about $150 US. It's worth it.