If you are looking for a dive shop on Ambergris, you'll be very happy with Amigos del Mar. While it's not a super "warm & fuzzy" group, the staff is extremely competent and professional, and they make every effort to ensure your dive is safe, enjoyable and easy. Not only do they put all of your equipment on the boat (and remember what equipment is yours), they set it up, bring it to you before you roll/stride off the boat, and even help you put it on. In fact, they even wash your mask for you (which I've never seen before).
Not a huge downside, but I sometimes found the groups to be a bit big for one divemaster (though Tony - the divemaster who handles the local dives - does a fantastic job of keeping track of everyone) and on night dives you'll be in a much smaller group. (and speaking of night dives, be sure to do it when it Ambergris - one of my favorite dives ever)
Enjoy your trip!