Alex,
There are a couple of dive centers in Ixtapa and Zihuatanejo. My wife and I dove with two of them on several ocasions during trips in 2001 and 2002. Our favorite operator was Ixtapa Divers. They were located in the Ixtapa hotel zone, in a strip mall across from the Barcelo. The DM/Guide was Roel and he was very adept at finding the little critters. Most of the time we dove with no more than 4 people and we were able to do some nice casual dives that ranged from 30 to 50 minutes (most sites we went to were less than 60 ft; Great for beginners). Although the shop was in Ixtapa, we would pick up the boat from the commercial pier in Zihuatanejo. Ocassionally we would actually meet the dive boat at the Marina in Ixtapa.
The other shop we dove with was the Zihuatanejo Scuba Center. I believe they were at a small hotel on Playa Las Gatas. They were referred to us by the dive shop we dove with several times in Acapulco. They seemed intent to get us in and out of the water in 30 minutes or less so we did not return to them. (Of course they might have changed that practice.)
One thing to keep in mind with Mexican Pacific Coast diving: If you want to see mounds of colorful coral, you won't find it here (applies not only to Ixtapa, but Pto Vallarta, Acapulco, Pto, Escondido & Huatulco all of which we've dove). What you will find is either rocky bottoms or underwater ridges smaller but similar to the ridges of the Mountains to the east. The fish school around the big outcroppings and hang out in between the big ridges (some of the ridges reminded me of big ships laying on their sides). In Ixt/Zihua there are plenty of bays and coves to explore were you can find seahorses, sea stars, octopus and many little fish hiding in the rocks as well as big schools of fish in the open. Dive site names that I can recall are Piedra Solitaria, El Chato, Zacatoso, Caleta de Chon & El Sacremento. (I could dig out my log books to see what others we dove if you'd like the info.)
The PADI Travel network has a few shops on their web site:
Zihuatanejo Dive Center: zihuatanejodivecenter dot com
Carlo Scuba: carloscuba dot com
Ixtapa Aqua Paradise: ixtapaaquaparadise dot com
Not sure about any of these folks but it should give you some starting places! Let me know if you need other info. You can find me here on occasion or the 4th Tuesday of every month at the Tritons Dive Club meetings.
Safe diving - Dave