If you're not certified then definitely see about that near home. You'll have far longer to learn and you'll meet people you may want to hang out/dive with later on after you're back. It doesn't matter what agency the dive shop works with, as all certifications are recognised here. If you want to do just the class work at home and all the water work here, or (more usual) the classwork and pool work there and just the open water dives here, then that can also be accommodated. Again, it doesn't matter whether the two shops are affiliated to the same agency or not, though it does simplify things a bit if they are. PADI is by far the most common agency world-wide and dominates here.
A variant is to do the theory at home but not with a dive center there but on your own, either by private arrangement with a dive center here of your choice, or to use the new PADI on-line learning facility which you can find details of at
www.padi.com. Despite the hi-tech aspects this doesn't really add much over the older distance learning system, under which you buy a manual locally, read it, and send tests etc by email to your chosen dive center.
Otherwise you can leave everything until you get here and that can work, but it's not the best way to learn and it'll eat up a chunk of your vacation.
Should have said - if you aren't currently a certified diver then DON'T buy a housing for your SLR. You won't know what you're doing and you'll waste a lot of money. If you already have a compact see if you can get a housing for it, or maybe think about getting the two new together. You shouldn't think of taking a camera down until you're at least reasonably competent underwater.
AmbergrisCaye.com 
