I dove in Zihua with my boyfriend and his daughter on March 17, 18, and 19, and the viz tended to vary. For example, on the last dive at Manzanillo, at times it would be 30 feet, then suddenly as we went along, drop to 5. We're mostly quarry divers so low viz doesn't phase us.
Travelure said it correctly: it's not like diving the Carribbean side of Mexico. I have to admit that I've never done that, yet I've heard it's warm water, very clear, etc, quite different from Zihua.
Zihua does have its charms, as do all dive sites. Different doesn't equal bad.
We wanted to dive with a variety of dive ops, however: Carlo was booked solid due to cruise ships, and Nautilus Divers never responded to many e-mails or phone calls. So, March 19th we went out with Zihuatanejo Dive Center (ZDC) to La Caleta de Chon, and Las Gatas. Jorge, the DM/instructor, was really cool, and while the viz at Las Gatas was bad, we still had 2 great dives. He treated us like the competent divers we are, which was not the case with Aqua Paradise.
On March 18, we went out with Aqua Paradise for 4 dives: 2 in the morning, and 2 at night. Although there were 2 instructors (I use the term loosely, we were not students) and 8 divers, instead of breaking us into 2 groups of 4, we had to go in one big group. That was not enjoyable. I watch my buoyancy and trim, yet I was forever getting banged into by other divers, kicked, etc. Also, the manner of the "instructor" was rather condescending--he tended to act as if we were students. They took our gear bags off the boat and put them in their dive shop in Ixtapa (yes, the boats were small, so I understand that) so when it came to the night dive, we didn't have our lights which we had left in the bags, not needing them for the morning dives (of course they let us use theirs, so what in the heck did I haul that thing from Indiana for? GRR!!)
We dove El Zacatoso and La Caleta de Chon (again!) on the morning dives, and in the evening did Piedra Solitaria and La Caleta (again!). Piedra Solitaria was very nice, lots of marine life. La Caleta in the morning was cool because we saw seahorses! And on the night dive, we ran across a tiny octopus there.
Wednesday the 19th we went out again with ZDC. We went back to Piedra Solitaria, which was even more fab in the daylight, then a place called Manzanillo (near la Caleta de Chon). All of us agreed that diving with ZDC was much more pleasant than diving with AP because Jorge treated us like fellow divers, not like idiots who had to have their hands held as they dove. It was much more relaxed, and a much less stressful atmosphere than with AP. With 8 divers on the AP boat, the instructors had to scramble to get gear set up for us, it was kind of tense. Also, I've never had any kind of "funny-tasting" air, but on 3 of the 4 dives with did with AP, the air had a weird taste to it.
Christian, the owner of ZDC, was very nice. We had to check out of our hotel before we went to the shop that Wed. morning, and she let us put all our luggage upstairs above the shop in her apartment the entire day. She even kept the shop open late in order for all 3 of us to shower and change late afternoon before boarding a bus to go back to Mexico City. We just felt that ZDC went above and beyond in the way we were treated.
There was a post somewhere from a few years ago about ZDC, very negative, and I should mention that ZDC is under new ownership now since a few months ago, and was nothing like what was stated in that post.
If you like cattle boats, then go for Aqua Paradise. Otherwise, my recommendation is Zihuatanejo Dive Center.
Happy diving!