We just got back from Ambergris yesterday. Our two teens did the classroom and pool class at home and did the open water portions with Happy and Grumpy. We HIGHLY recommend them. They mainly do instruction for PADI, and not a dive shop. What we liked about them is they were private, you just need two people to do it privatily. They have a very good boat, and there equipment is in good condition. Where ever you dive you may want references from people who had dive lately, all dive shops are not the same down there. They will pick you up at your hotel, it is two of them working with you, and they are thorough, don't skip a thing. They are on the main web page under Happy and Grumpy, and also under the dive portion of the web site.
Good Luck and enjoy diving.