I have visited Belize and its barrier Reef a few times for scuba trips, and its definitely worth a visit. It is less touristy then nearby Mexico, yet has some amazing diving. During the right season you can see whale sharks in the south, and I saw a few white tip sharks in the Blue Hole as well. You have to be careful what dive center you go with, as many in Belize have a bad reputation for being too lax with the rules or dive safety. 

Another fun thing to do with your dive operator is the now popular lion fish hunts. They give you little Hawaiian style spear guns, and you get to shoot this invasive species all day. Pretty fun and helps the environment.