I don't know of any guides, but I went about a year ago and some local yahoo was recruiting tourists along the beach. My wife and I and a couple we were with decided to go for it. It was rediculous.... It was two kids may 16 or 17 in a 19 foot wooden boat with a small glass bottom. We start out cruising over the reef and they are looking through the glass in the bottom of the boat. Turns out they were looking for the reef fish. All of a sudden they stop the boat and pass around the rods and tell us to get after it. We do catch fish but they are all the tropical reef fish like Parrots and angels and all are about 8 to 10 inches long. This was their idea of fishing. To top it all off we were violating the international wildlife preserve the entire time.
Admittedly we did not expect much from the guys and ended up just getting drunk(I negotiated a couple cases of beer into the deal for free) and riding around in their boat.
Anyway, Check with your hotel. If it's a reputable hotel they can refer you to someone or check with one of the local dive shops. I used "Studio Blue Dive Center" when I was there. The guy running it at the time was a French guy. He was real cool and found him to be sincere in what he recommended.(plus their ball caps look cool).
If your into SCUBA, you have to go on a cenote dive(underwater cavern). My wife and I did it and it was cool(fresh water not salty).