The Fiesta Americana is a nice place and there are many nice places for divers to stay in Coz. Plaza Las Glorias, Brisas, the El Cozumeleno, El Presidente and Caribe Blu (formerly the Lorena) are some other nice resorts you may want to investigate. It would be difficult to go too far wrong with any of them and there are numerous others worthy of consideration. Find one that seems to fit your price range and dive in.
There are also many very reputable and enjoyable dive operations down there each of which seems to attract their own unique following of fans. Aldora, Dive Paradise, Papa Hogs, Dive Palancar, Sand Dollar and Blue Bubble are just a few reputable dive operators that come to mind. My favorite is Aldora for a variety of reasons but all of the operators I've mentioned have their own feel and good reputations. A great time can be had diving with any of the above mentioned operators.
A bit of searching on these boards and on web searches should give you plenty of ideas and opinions to consider. I hope having a few names will help you along with your investigations.
Coz is a great place, enjoy!