We dove with Hustler Tours on our last visit to AC, and were very please with their service. Now, after nearly 100 dives in 5 years, we have decided to go for our Advance Open Water Certification, and again ,we plan to use Hustler. But, from the posts on the board, it sounds like most of the shops are reputable and provide a safe instruction. The cost of certification is about the same with all of the services. The important thing is that you are confortable with the dive shop/instructor. There has been much discussion recently about which accrediting service (PADI, NAUI, or SSI) to use, and again the most important thing is that you have confidence in your divemaster and instructor.
Another option is to take the Open Water Certification course in your home town; most dive shops and YMCA's offer this as well.
Whatever you do, make sure that you take that course, because the experience of diving is like no other on this earth! The feeling of weighlessness in the water and all of the sights to be seen underwater. From the many varied types of coral and sponge to the infinite variety of fish to spot underwater, it is truely remarkable. Those that never experience it are missing an entirely different world underwater that is absolutely awesome.