Wow, really want to know how your certification went. I have been to Belize and Austrilia. Dove them both. Loved Great Barrier Reef.
On the cruise I went on Belize had the roughest water but none of us got sea sick (all got sun burnt). Be prepared to see great sights but don't expect the surroundings to be USA. Also, don't forget about the blue hole, something special Belize has. I've never seen it but imagine that it's great.
Have fun!