Agree with Rich about getting certified at home. It will be a more thorough process at home and probably cheaper.
I LOVE diving Belize. I highly recommend Hopkins Underwater Adventures in Hopkins, Belize. It’s located on the beach, about 5 minutes from the village of Hopkins. Great staff and very reasonable. Not the easiest place to get to though.
3 hour drive from Belize International or you can take a smaller prop plane from Belize International to Dangriga and then get a 30 minute taxi to Hopkins.