I recommend you buy your mask and snorkel before you go down. You will need help making sure you get a good fit and you should be able to try out a few in the dive shop's pool before you buy. I never saw any dive masks for sale at the dive ops I was at. That doesn't mean no one sells them but I did think it odd that none of the operator's shops I was in seemed to be carrying any retail gear.

Perhaps Blue Hole Divers sells them? Anybody know?

You will be able to rent mask and snorkel along with the rest of your gear if you don't buy before you go. However, if you are serious about this take an hour or two at the dive shop in your home town before you leave. Make sure you try it with the snorkel in your mouth. You will be happy that you have your own.

As for recommendations for Ambergris Divers, I don't believe I've ever seen anyone recommend their instructor(s). That doesn't mean that there is anything wrong with him(them) just that I haven't seen anyone make a recommendation in that regard. Whereas I have seen recommendations for the instructors at Amigos and at Barefoot. If nothing has changed the instructor at Amigos is an English fella. And they have their own classroom area, separate from the shop. If I'm recalling correctly, I believe that you will be closer to Amigo's if you are staying at Rubies, than you would be to Barefoot.

A fish and a bird can fall in love, but where will they build their nest?