I got back from Mazatlan last night. Although I took my equipment down, I didn't get into the the water. I had an odd feeling that I really shouldn't try to push a couple dives into a 48 hour trip even though I'd be within DAN's guidelines on flying after diving.
I found a couple dive shops in the phone book, but one had a reference to rescue work, so I really wondered about it. There are apparently a couple of PADI operators down there, but they work primarily through the resorts on request. When I asked the concierge about diving tours, she told me that they could call an instructor who would provide equipment and take me out. Cost was $60 a head, two person minimum. I was told that I could catch a boat at El Cid (a resort) out to Deer Island (just offshore) and could dive to around 30 feet. However, that would be a "bring your own kit" deal.
I also talked to a couple of locals about the diving. We were fortunate that one of the real estate brokers we're dealing with is an avid surfer, so he knows about the waters in the area. He told me that diving was really hit-and-miss. He said that the water around Mazatlan is fairly shallow out to about 22 miles offshore, where it drops off into blue water. That's where they do the marlin and and other sportfishing. He said that the bottom composition near the shore is such that it gets blown out very easily. He said that Monday (which was a little overcast with a very slight breeze) was not a divable day, and that the slightest bit of current or breeze trashes visibility. Therefore, it's extremely difficult for people to plan dives ahead of time--he said divers pretty much look at the weather in the morning and make a decision to dive or pass. His opinion was that Mazatlan is definitely not a diving destination, especially since Acapulco and Puerto Vallarta offer much better conditions.
On the other hand, the snorkeling is supposed to be good. I think the final consensus is to avoid Mazatlan if you're going for the diving. Mazatlan is a charming city that is extremely tourist friendly. If you want a place to hang out for a week and decompress, it's perfect. And it has a good night life. Of course, since there's water, you can dive, but it ain't Bonaire.