Was in that region some time ago, before the massive development, and even then the diving was only so-so. Get periodic updates from sources & things not surprisingly haven't improved. Still, if you have a strong urge to submerge, I'd take your gear. If you can get to the more distant sites, there's a slim (very) chance of seeing a manta, and there is some small marine life on the sparse patch reef.
Forget Ixtapa diving---snorkeling off the Club Med beach is better. Go to Zihuatanejo to dive. Best ops in town are old (like real old by now) Owen Lee at Las Gatas and an ecologically-minded shop run by a chap with a strong marine biology background called Zihuatanejo Scuba Center.
Sorry that I couldn't be more enthusiastic.
DocVikingo